diff --git a/types/reflect.go b/types/reflect.go
index def5567..83f9c89 100644
--- a/types/reflect.go
+++ b/types/reflect.go
@@ -23,13 +23,9 @@ func deserializeType(d *Deserializer) reflect.Type {
 func serializeAny(s *Serializer, t reflect.Type, p unsafe.Pointer) {
 	if serde, ok := s.serdes.serdeOf(t); ok {
 		offset := len(s.b)
-		s.b = append(s.b, 0, 0, 0, 0) // store a 32-bit size placeholder
+		s.b = append(s.b, 0, 0, 0, 0, 0, 0, 0, 0) // store a 64-bit size placeholder
 		serde.ser(s, t, p)
-		size := len(s.b) - offset
-		if int64(size) > math.MaxUint32 {
-			panic("overflow")
-		}
-		binary.LittleEndian.PutUint32(s.b[offset:], uint32(size))
+		binary.LittleEndian.PutUint64(s.b[offset:], uint64(len(s.b)-offset))
 		return
 	}
 
@@ -102,7 +98,7 @@ func serializeAny(s *Serializer, t reflect.Type, p unsafe.Pointer) {
 
 func deserializeAny(d *Deserializer, t reflect.Type, p unsafe.Pointer) {
 	if serde, ok := d.serdes.serdeOf(t); ok {
-		d.b = d.b[4:] // skip size prefix
+		d.b = d.b[8:] // skip size prefix
 		serde.des(d, t, p)
 		return
 	}