diff --git a/union.go b/union.go
index 031e84f..ac5a18f 100644
--- a/union.go
+++ b/union.go
@@ -327,7 +327,7 @@ func buildCodecForTypeDescribedBySliceTwoWayJSON(st map[string]*Codec, enclosing
 }
 
 func checkAll(allowedTypes []string, cr *codecInfo, buf []byte) (interface{}, []byte, error) {
-	for _, name := range cr.allowedTypes {
+	for _, name := range allowedTypes {
 		if name == "null" {
 			// skip null since we know we already got type float64
 			continue
@@ -344,6 +344,14 @@ func checkAll(allowedTypes []string, cr *codecInfo, buf []byte) (interface{}, []
 	}
 	return nil, buf, fmt.Errorf("could not decode any json data in input %v", string(buf))
 }
+
+// sortedCopy returns a new slice that is a sorted copy of the provided types.
+func sortedCopy(allowedTypes []string) []string {
+	local := make([]string, len(allowedTypes))
+	copy(local, allowedTypes)
+	sort.Strings(local)
+	return local
+}
 func nativeAvroFromTextualJSON(cr *codecInfo) func(buf []byte) (interface{}, []byte, error) {
 	return func(buf []byte) (interface{}, []byte, error) {
 
@@ -398,18 +406,14 @@ func nativeAvroFromTextualJSON(cr *codecInfo) func(buf []byte) (interface{}, []b
 			// longNativeFromTextual
 			// int
 			// intNativeFromTextual
-
-			// sorted so it would be
-			// double, float, int, long
-			// that makes the priorities right by chance
-			sort.Strings(cr.allowedTypes)
+			allowedTypes = sortedCopy(allowedTypes)
 
 		case map[string]interface{}:
 
 			// try to decode it as a map
 			// because a map should fail faster than a record
 			// if that fails assume record and return it
-			sort.Strings(cr.allowedTypes)
+			allowedTypes = sortedCopy(allowedTypes)
 		}
 
 		return checkAll(allowedTypes, cr, buf)
diff --git a/union_test.go b/union_test.go
index b66884f..9f87f97 100644
--- a/union_test.go
+++ b/union_test.go
@@ -339,3 +339,38 @@ func TestUnionJson(t *testing.T) {
 	testNativeToTextualJSONPass(t, `{"type":"record","name":"kubeEvents","fields":[{"name":"field1","type":"string","default":""},{"name":"field2","type":"string"}]}`, map[string]interface{}{"field1": "", "field2": "deef"}, []byte(`{"field1":"","field2":"deef"}`))
 
 }
+
+func TestStandardJSONFull_SimpleUnionMutationWouldMislabel(t *testing.T) {
+	// Minimal repro: union ["null","long"]. If allowedTypes were mutated (sorted)
+	// after a textual decode, the subsequent binary decode could label index 1 as "null".
+	codec, err := NewCodecForStandardJSONFull(`["null","long"]`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Trigger textual path that previously sorted cr.allowedTypes.
+	if _, _, err := codec.NativeFromTextual([]byte("1")); err != nil {
+		t.Fatalf("textual decode failed: %v", err)
+	}
+	// Binary for union index 1 (long) with value 3: 0x02 0x06
+	datum, rest, err := codec.NativeFromBinary([]byte{0x02, 0x06})
+	if err != nil {
+		t.Fatalf("binary decode failed: %v", err)
+	}
+	if len(rest) != 0 {
+		t.Fatalf("unexpected trailing bytes: %d", len(rest))
+	}
+	m, ok := datum.(map[string]interface{})
+	if !ok {
+		t.Fatalf("expected union map, got %T", datum)
+	}
+	if _, bad := m["null"]; bad {
+		t.Fatalf("mis-labeled union: got key 'null', want 'long': %v", m)
+	}
+	v, ok := m["long"]
+	if !ok {
+		t.Fatalf("missing 'long' key: %v", m)
+	}
+	if v.(int64) != int64(3) {
+		t.Fatalf("wrong value: got %v want %v", v, int64(3))
+	}
+}