diff --git a/rpc_util.go b/rpc_util.go
index 4be93ff5..46a6801b 100644
--- a/rpc_util.go
+++ b/rpc_util.go
@@ -160,7 +160,7 @@ func (p *parser) recvMsg() (pf payloadFormat, msg []byte, err error) {
 // generates the message header of 0 message length.
 func encode(c Codec, msg interface{}, pf payloadFormat) ([]byte, error) {
 	var b []byte
-	var length uint32
+	var length uint
 	if msg != nil {
 		var err error
 		// TODO(zhaoq): optimize to reduce memory alloc and copying.
@@ -168,7 +168,7 @@ func encode(c Codec, msg interface{}, pf payloadFormat) ([]byte, error) {
 		if err != nil {
 			return nil, err
 		}
-		length = uint32(len(b))
+		length = uint(len(b))
 	}
 	if length > math.MaxUint32 {
 		return nil, Errorf(codes.InvalidArgument, "grpc: message too large (%d bytes)", length)
@@ -184,7 +184,7 @@ func encode(c Codec, msg interface{}, pf payloadFormat) ([]byte, error) {
 	// Write payload format
 	buf[0] = byte(pf)
 	// Write length of b into buf
-	binary.BigEndian.PutUint32(buf[1:], length)
+	binary.BigEndian.PutUint32(buf[1:], uint32(length))
 	// Copy encoded msg to buf
 	copy(buf[5:], b)