diff --git a/src/BLite.Core/Collections/DocumentCollection.cs b/src/BLite.Core/Collections/DocumentCollection.cs
index 95189124..f387bf12 100644
--- a/src/BLite.Core/Collections/DocumentCollection.cs
+++ b/src/BLite.Core/Collections/DocumentCollection.cs
@@ -902,7 +902,8 @@ internal DocumentCollection(StorageEngine storage, ITransactionHolder transactio
 
     /// <summary>
     /// Decimal properties used to be indexed as <c>(double)value</c>. An index whose persisted
-    /// <see cref="IndexMetadata.KeyFormat"/> predates the exact decimal encoding, or a full-text
+    /// <see cref="IndexMetadata.KeyFormat"/> predates the exact decimal encoding, a double-keyed
+    /// index holding a key of -0.0 from before -0.0 and +0.0 shared a key (#249), or a full-text
     /// index written in another format or holding keys it did not write, is rebuilt from the
     /// documents into a copy that queries don't see, the first time the collection is opened. Only
     /// after the copy committed is it swapped in, with its target format, in one metadata save, and
diff --git a/src/BLite.Core/Indexing/CollectionSecondaryIndex.cs b/src/BLite.Core/Indexing/CollectionSecondaryIndex.cs
index 887135d0..ed40ad25 100644
--- a/src/BLite.Core/Indexing/CollectionSecondaryIndex.cs
+++ b/src/BLite.Core/Indexing/CollectionSecondaryIndex.cs
@@ -805,18 +805,43 @@ private IndexKey ConvertToIndexKey(object value)
 
     private bool? _isDecimalKeyed;
 
-    private bool IsDecimalKeyed => _isDecimalKeyed ??= ComputeIsDecimalKeyed();
+    private bool IsDecimalKeyed => _isDecimalKeyed ??= SingleKeyType() == typeof(decimal);
 
-    private bool ComputeIsDecimalKeyed()
+    /// <summary>The type of the single indexed property (nullable unwrapped), or null for a compound key.</summary>
+    private Type? SingleKeyType()
     {
-        if (_definition.PropertyPaths.Length != 1) return false;
+        if (_definition.PropertyPaths.Length != 1) return null;
         // Only the boxing conversion to object is looked through: an explicit cast in the selector
         // (e.g. (double)p.Price) changes the key type on purpose and is encoded as that type.
         var body = _definition.KeySelectorExpression.Body;
         while (body is System.Linq.Expressions.UnaryExpression { NodeType: System.Linq.Expressions.ExpressionType.Convert } u
                && u.Type == typeof(object))
             body = u.Operand;
-        return (Nullable.GetUnderlyingType(body.Type) ?? body.Type) == typeof(decimal);
+        return Nullable.GetUnderlyingType(body.Type) ?? body.Type;
+    }
+
+    // ── Double-keyed indexes ──────────────────────────────────────────────────
+    // IndexKey(double) gives -0.0 the key of +0.0 (#249). Earlier releases keyed -0.0 apart, just
+    // below +0.0; such an entry is neither found by a query nor removed when its document is updated
+    // or deleted, so a double- or float-keyed index that still holds one is rebuilt on open
+    // (NeedsKeyFormatMigration). Looking for it costs one seek per index and open.
+
+    // The former key of -0.0: every bit of 0x8000_0000_0000_0000 inverted, after KeyPrefix.
+    private static readonly IndexKey s_legacyNegativeZeroKey =
+        new IndexKey(new byte[] { 0x7F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF });
+
+    private bool HoldsLegacyNegativeZeroKey()
+    {
+        if (_btreeIndex == null) return false;
+        var keyType = SingleKeyType();
+        if (keyType != typeof(double) && keyType != typeof(float)) return false;
+
+        foreach (var _ in _btreeIndex.Range(
+                     CreateCompositeKeyBoundary(s_legacyNegativeZeroKey, useMinObjectId: true),
+                     CreateCompositeKeyBoundary(s_legacyNegativeZeroKey, useMinObjectId: false),
+                     IndexDirection.Forward, null))
+            return true;
+        return false;
     }
 
     private static decimal ToDecimalClamped(double d)
@@ -848,10 +873,12 @@ private static decimal ToDecimalClamped(float f)
     /// True when this B-tree index keys a <c>decimal</c> property and its pages still use the legacy
     /// format, i.e. hold <c>(double)value</c> keys that the exact decimal encoding cannot match; or
     /// when this full-text index was written with another analyzer version or holds keys it did not
-    /// write (e.g. from an older BLite that read it as a plain B-tree).
+    /// write (e.g. from an older BLite that read it as a plain B-tree); or when this double- or
+    /// float-keyed B-tree index holds a key of -0.0 written before -0.0 and +0.0 shared a key.
     /// </summary>
     internal bool NeedsKeyFormatMigration =>
         (_btreeIndex != null && KeyFormat < IndexMetadata.CurrentKeyFormat && IsDecimalKeyed)
+        || HoldsLegacyNegativeZeroKey()
         || (_fullTextIndex != null && (KeyFormat != FullTextIndex.FormatVersion || _fullTextIndex.HasForeignKeys()));
 
     /// <summary>The key format a rebuilt copy of this index is written in.</summary>
diff --git a/src/BLite.Core/Indexing/IndexKey.cs b/src/BLite.Core/Indexing/IndexKey.cs
index 35b2269f..a36926c0 100644
--- a/src/BLite.Core/Indexing/IndexKey.cs
+++ b/src/BLite.Core/Indexing/IndexKey.cs
@@ -97,6 +97,8 @@ public IndexKey(double value)
         // Positive: flip sign bit only  → 1xxx…  (positive doubles sort after all negatives)
         // Negative: flip all bits        → sort from least-negative first
         // Result: lexicographic byte order == IEEE 754 numeric order for all finite and ±Inf.
+        // -0.0 == +0.0, so both take the key of +0.0; -0.0 would otherwise encode just below it (#249).
+        if (value == 0) value = 0.0;
         var bits = BitConverter.DoubleToInt64Bits(value);
         ulong u = bits < 0 ? ~(ulong)bits : (ulong)bits | 0x8000_0000_0000_0000UL;
         _data = [KeyPrefix, (byte)(u >> 56), (byte)(u >> 48), (byte)(u >> 40), (byte)(u >> 32),
diff --git a/tests/BLite.Shared/SignedZeroContexts.cs b/tests/BLite.Shared/SignedZeroContexts.cs
new file mode 100644
index 00000000..f1b16196
--- /dev/null
+++ b/tests/BLite.Shared/SignedZeroContexts.cs
@@ -0,0 +1,43 @@
+using BLite.Bson;
+using BLite.Core;
+using BLite.Core.Collections;
+using BLite.Core.Metadata;
+
+namespace BLite.Shared;
+
+// Contexts for SignedZeroIndexTests (#249): the same collection with and without indexes on a
+// double and a float property.
+
+public sealed class SzDoc
+{
+    public ObjectId Id { get; set; }
+    public string Name { get; set; } = "";
+    public double Ratio { get; set; }
+    public float Level { get; set; }
+}
+
+/// <summary>Indexes on <see cref="SzDoc.Ratio"/> and <see cref="SzDoc.Level"/>.</summary>
+public partial class SzIndexedContext : DocumentDbContext
+{
+    public DocumentCollection<ObjectId, SzDoc> Docs { get; set; } = null!;
+    public SzIndexedContext(string path) : base(path) => InitializeCollections();
+
+    protected override void OnModelCreating(ModelBuilder modelBuilder)
+    {
+        modelBuilder.Entity<SzDoc>().ToCollection("sz_docs")
+            .HasIndex(x => x.Ratio)
+            .HasIndex(x => x.Level);
+    }
+}
+
+/// <summary>The same collection without secondary indexes.</summary>
+public partial class SzPlainContext : DocumentDbContext
+{
+    public DocumentCollection<ObjectId, SzDoc> Docs { get; set; } = null!;
+    public SzPlainContext(string path) : base(path) => InitializeCollections();
+
+    protected override void OnModelCreating(ModelBuilder modelBuilder)
+    {
+        modelBuilder.Entity<SzDoc>().ToCollection("sz_docs");
+    }
+}
diff --git a/tests/BLite.Tests/SignedZeroIndexTests.cs b/tests/BLite.Tests/SignedZeroIndexTests.cs
new file mode 100644
index 00000000..1bc82c79
--- /dev/null
+++ b/tests/BLite.Tests/SignedZeroIndexTests.cs
@@ -0,0 +1,202 @@
+using System.Linq.Expressions;
+using BLite.Bson;
+using BLite.Core.Indexing;
+using BLite.Core.Query;
+using BLite.Core.Storage;
+
+namespace BLite.Tests;
+
+/// <summary>
+/// Regression tests for issue #249: <c>IndexKey(double)</c> gave -0.0 and +0.0 different keys, so
+/// an index on a double missed -0.0 for <c>== 0.0</c> and <c>&gt;= 0.0</c> and found it for
+/// <c>&lt; 0.0</c>, unlike IEEE 754 equality, which C# and the plain collection use. Each query must
+/// agree with LINQ-to-Objects over the same documents, with and without the indexes.
+/// </summary>
+public class SignedZeroIndexTests : IAsyncLifetime
+{
+    private readonly string _indexedPath = Path.Combine(Path.GetTempPath(), $"sz_indexed_{Guid.NewGuid():N}.db");
+    private readonly string _plainPath = Path.Combine(Path.GetTempPath(), $"sz_plain_{Guid.NewGuid():N}.db");
+    private readonly string _legacyPath = Path.Combine(Path.GetTempPath(), $"sz_legacy_{Guid.NewGuid():N}.db");
+    private SzIndexedContext _indexed = null!;
+    private SzPlainContext _plain = null!;
+
+    private static SzDoc[] Seed() =>
+    [
+        new SzDoc { Name = "pz", Ratio = 0.0, Level = 0f },
+        new SzDoc { Name = "a", Ratio = -2.5, Level = -2.5f },
+        new SzDoc { Name = "nz", Ratio = -0.0, Level = -0f },
+        new SzDoc { Name = "b", Ratio = 1.5, Level = 1.5f },
+        new SzDoc { Name = "c", Ratio = -1.0, Level = -1f },
+        new SzDoc { Name = "d", Ratio = 3.0, Level = 3f },
+    ];
+
+    public async Task InitializeAsync()
+    {
+        _indexed = new SzIndexedContext(_indexedPath);
+        _plain = new SzPlainContext(_plainPath);
+        await _indexed.Docs.InsertBulkAsync(Seed());
+        await _plain.Docs.InsertBulkAsync(Seed());
+    }
+
+    public Task DisposeAsync()
+    {
+        _indexed.Dispose();
+        _plain.Dispose();
+        foreach (var path in new[] { _indexedPath, _plainPath, _legacyPath })
+        {
+            if (File.Exists(path)) File.Delete(path);
+            var wal = Path.ChangeExtension(path, ".wal");
+            if (File.Exists(wal)) File.Delete(wal);
+        }
+        return Task.CompletedTask;
+    }
+
+    [Fact]
+    public void IndexKey_Of_NegativeZero_Is_The_Key_Of_PositiveZero()
+    {
+        Assert.Equal(new IndexKey(0.0), new IndexKey(-0.0));
+        Assert.True(new IndexKey(-double.Epsilon) < new IndexKey(-0.0));
+        Assert.True(new IndexKey(-0.0) < new IndexKey(double.Epsilon));
+    }
+
+    [Fact]
+    public async Task Equality_Finds_Both_Zeros()
+    {
+        await AssertAgreesAsync(x => x.Ratio == 0.0, "nz", "pz");
+        await AssertAgreesAsync(x => x.Ratio == -0.0, "nz", "pz");
+        await AssertAgreesAsync(x => x.Level == 0f, "nz", "pz");
+    }
+
+    [Fact]
+    public async Task Ranges_Treat_Both_Zeros_Alike()
+    {
+        await AssertAgreesAsync(x => x.Ratio >= 0.0, "b", "d", "nz", "pz");
+        await AssertAgreesAsync(x => x.Ratio > -0.0, "b", "d");
+        await AssertAgreesAsync(x => x.Ratio < 0.0, "a", "c");
+        await AssertAgreesAsync(x => x.Ratio <= -0.0, "a", "c", "nz", "pz");
+        await AssertAgreesAsync(x => x.Ratio >= -1.0 && x.Ratio <= 0.0, "c", "nz", "pz");
+    }
+
+    [Fact]
+    public async Task Count_Finds_Both_Zeros()
+    {
+        foreach (var docs in new[] { _indexed.Docs, _plain.Docs })
+        {
+            Assert.Equal(2, await docs.AsQueryable().CountAsync(x => x.Ratio == 0.0));
+            Assert.Equal(4, await docs.AsQueryable().CountAsync(x => x.Ratio >= 0.0));
+            Assert.Equal(2, await docs.AsQueryable().CountAsync(x => x.Ratio < 0.0));
+        }
+    }
+
+    [Fact]
+    public async Task OrderBy_Keeps_Both_Zeros_Together()
+    {
+        foreach (var (label, docs) in new[] { ("indexed", _indexed.Docs), ("plain", _plain.Docs) })
+        {
+            var all = await docs.FindAllAsync().ToListAsync();
+
+            // The two zeros are equal, so their order within the window is not defined; the window
+            // over them holds both.
+            var window = (await docs.AsQueryable().OrderBy(x => x.Ratio).Skip(2).Take(2).ToListAsync()).Select(x => x.Name);
+            Assert.Equal(["nz", "pz"], window.Order(StringComparer.Ordinal));
+
+            var nonNegative = (await docs.AsQueryable().Where(x => x.Ratio >= 0.0).OrderBy(x => x.Ratio).ToListAsync())
+                .Select(x => x.Ratio).ToArray();
+            Assert.Equal(4, nonNegative.Length);
+            Assert.True(nonNegative.Zip(new[] { 0.0, 0.0, 1.5, 3.0 }).All(p => p.First == p.Second), label);
+
+            var descending = (await docs.AsQueryable().OrderByDescending(x => x.Ratio).ToListAsync()).Select(x => x.Ratio).ToArray();
+            Assert.True(descending.Zip(all.OrderByDescending(x => x.Ratio).Select(x => x.Ratio)).All(p => p.First == p.Second), label);
+        }
+    }
+
+    /// <summary>
+    /// An index written before the fix keeps -0.0 under its former key, which the new bounds never
+    /// reach and which an update or delete of the document no longer removes. Opening the
+    /// collection rebuilds such an index; one without a -0.0 key is left alone.
+    /// </summary>
+    [Fact]
+    public async Task Index_Holding_The_Former_NegativeZero_Key_Is_Rebuilt_On_Open()
+    {
+        uint levelRoot;
+        using (var db = new SzIndexedContext(_legacyPath))
+        {
+            await db.Docs.InsertBulkAsync(Seed());
+            var index = RatioIndex(db);
+            var negativeZero = (await db.Docs.FindAllAsync().ToListAsync()).Single(x => x.Name == "nz");
+
+            // Recreate what an earlier release wrote for -0.0: [0x02 7F FF FF FF FF FF FF FF][id].
+            var locations = index.BTreeIndex!.Range(IndexKey.NullSentinelNext, IndexKey.MaxKey).ToList();
+            var current = locations.Single(e => e.Key.Data.StartsWith(new IndexKey(-0.0).Data)
+                                                && e.Key.Data.EndsWith(new IndexKey(negativeZero.Id).Data));
+            var legacyUserKey = new byte[] { 0x7F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
+            var payload = legacyUserKey.Concat(new IndexKey(negativeZero.Id).Data.ToArray()).ToArray();
+            using (var txn = db.BeginTransaction())
+            {
+                Assert.True(index.BTreeIndex.Delete(current.Key, current.Location, txn.TransactionId));
+                index.BTreeIndex.Insert(new IndexKey(payload), current.Location, txn.TransactionId);
+                await txn.CommitAsync();
+            }
+            Assert.Contains(AllKeys(index), k => k.Data.StartsWith(new IndexKey(legacyUserKey).Data));
+            levelRoot = LevelIndex(db).RootPageId;
+        }
+
+        uint ratioRoot;
+        using (var db = new SzIndexedContext(_legacyPath))
+        {
+            var index = RatioIndex(db);
+            Assert.DoesNotContain(AllKeys(index), k => k.Data[1] == 0x7F);
+            Assert.Equal(6, AllKeys(index).Count);
+            Assert.Equal(levelRoot, LevelIndex(db).RootPageId);
+
+            Assert.Equal(["nz", "pz"], (await db.Docs.AsQueryable().Where(x => x.Ratio == 0.0).ToListAsync())
+                .Select(x => x.Name).Order(StringComparer.Ordinal));
+
+            // The entry now goes away with its document.
+            var nz = (await db.Docs.FindAllAsync().ToListAsync()).Single(x => x.Name == "nz");
+            Assert.True(await db.Docs.DeleteAsync(nz.Id));
+            Assert.Equal(5, AllKeys(index).Count);
+            ratioRoot = index.RootPageId;
+        }
+
+        using (var db = new SzIndexedContext(_legacyPath))
+        {
+            // Nothing left to rebuild.
+            Assert.Equal(ratioRoot, RatioIndex(db).RootPageId);
+            Assert.Equal(levelRoot, LevelIndex(db).RootPageId);
+            Assert.Equal(["pz"], (await db.Docs.AsQueryable().Where(x => x.Ratio == 0.0).ToListAsync()).Select(x => x.Name));
+        }
+    }
+
+    // ─── Helpers ─────────────────────────────────────────────────────────────
+
+    private static CollectionSecondaryIndex<ObjectId, SzDoc> RatioIndex(SzIndexedContext db)
+        => db.Docs.IndexManager.GetAllIndexes().Single(i => i.PropertyPaths.Single() == nameof(SzDoc.Ratio));
+
+    private static CollectionSecondaryIndex<ObjectId, SzDoc> LevelIndex(SzIndexedContext db)
+        => db.Docs.IndexManager.GetAllIndexes().Single(i => i.PropertyPaths.Single() == nameof(SzDoc.Level));
+
+    private static List<IndexKey> AllKeys(CollectionSecondaryIndex<ObjectId, SzDoc> index)
+        => index.BTreeIndex!.Range(IndexKey.NullSentinelNext, IndexKey.MaxKey).Select(e => e.Key).ToList();
+
+    /// <summary>
+    /// Runs <paramref name="predicate"/> on the indexed and the plain collection and checks both
+    /// against LINQ-to-Objects over the documents as read back, and against <paramref name="expected"/>.
+    /// </summary>
+    private async Task AssertAgreesAsync(Expression<Func<SzDoc, bool>> predicate, params string[] expected)
+    {
+        foreach (var (label, docs) in new[] { ("indexed", _indexed.Docs), ("plain", _plain.Docs) })
+        {
+            var all = await docs.FindAllAsync().ToListAsync();
+            var reference = Names(all.Where(predicate.Compile()));
+            Assert.Equal(expected, reference);
+
+            var actual = Names(await docs.AsQueryable().Where(predicate).ToListAsync());
+            Assert.True(reference.SequenceEqual(actual),
+                $"{label}: {predicate} returned [{string.Join(", ", actual)}], LINQ-to-Objects [{string.Join(", ", reference)}]");
+        }
+    }
+
+    private static string[] Names(IEnumerable<SzDoc> docs)
+        => docs.Select(d => d.Name).Order(StringComparer.Ordinal).ToArray();
+}