diff --git a/uuid.go b/uuid.go
index dc75cee..685db49 100644
--- a/uuid.go
+++ b/uuid.go
@@ -23,9 +23,15 @@ type UUID [16]byte
 type Version byte
 
 // A Variant represents a UUID's variant.
+//
+// Any valid 128-bit UUID classifies into RFC4122, Reserved, Microsoft, or
+// Future. Invalid represents an uninitialized or non-UUID sentinel.
 type Variant byte
 
-// Constants returned by Variant.
+// Constants representing UUID variants.
+//
+// Any valid 128-bit UUID classifies into RFC4122, Reserved, Microsoft, or
+// Future. Invalid represents an uninitialized or non-UUID sentinel.
 const (
 	Invalid   = Variant(iota) // Invalid UUID
 	RFC4122                   // The variant specified in RFC9562(obsoletes RFC4122).
@@ -297,6 +303,10 @@ func encodeHex(dst []byte, uuid UUID) {
 }
 
 // Variant returns the variant encoded in uuid.
+//
+// Any valid 128-bit UUID classifies into RFC4122, Reserved, Microsoft, or
+// Future. Variant never returns Invalid, which represents an uninitialized or
+// non-UUID sentinel.
 func (uuid UUID) Variant() Variant {
 	switch {
 	case (uuid[8] & 0xc0) == 0x80:
diff --git a/uuid_test.go b/uuid_test.go
index 906ecbe..4340f82 100644
--- a/uuid_test.go
+++ b/uuid_test.go
@@ -60,6 +60,8 @@ var tests = []test{
 	{"f47ac10b-58cc-4372-e567-0e02b2c3d479", 4, Future, true},
 	{"f47ac10b-58cc-4372-f567-0e02b2c3d479", 4, Future, true},
 
+	// For invalid UUID strings, Parse returns an error and the test runner returns
+	// early without evaluating version or variant. Invalid is used as a placeholder.
 	{"f47ac10b158cc-5372-a567-0e02b2c3d479", 0, Invalid, false},
 	{"f47ac10b-58cc25372-a567-0e02b2c3d479", 0, Invalid, false},
 	{"f47ac10b-58cc-53723a567-0e02b2c3d479", 0, Invalid, false},
@@ -165,6 +167,31 @@ func TestConstants(t *testing.T) {
 	}
 }
 
+func TestVariantInvalid(t *testing.T) {
+	if s := Invalid.String(); s != "Invalid" {
+		t.Errorf("Invalid.String() = %q, want %q", s, "Invalid")
+	}
+	if s := Variant(0).String(); s != "Invalid" {
+		t.Errorf("Variant(0).String() = %q, want %q", s, "Invalid")
+	}
+}
+
+func TestVariantAllBytes(t *testing.T) {
+	for b := 0; b <= 0xff; b++ {
+		var u UUID
+		u[8] = byte(b)
+		v := u.Variant()
+		if v == Invalid {
+			t.Errorf("UUID with byte[8]=%#x returned Invalid variant", b)
+		}
+		switch v {
+		case RFC4122, Reserved, Microsoft, Future:
+		default:
+			t.Errorf("UUID with byte[8]=%#x returned unexpected variant %v", b, v)
+		}
+	}
+}
+
 func TestRandomUUID(t *testing.T) {
 	m := make(map[string]bool)
 	for x := 1; x < 32; x++ {