Type sign fix for C#
diff --git a/src/csharp/ext/grpc_csharp_ext.c b/src/csharp/ext/grpc_csharp_ext.c
index 70c0fbc..51e0728 100644
--- a/src/csharp/ext/grpc_csharp_ext.c
+++ b/src/csharp/ext/grpc_csharp_ext.c
@@ -252,7 +252,7 @@
   if (!ctx->recv_message) {
     return -1;
   }
-  return grpc_byte_buffer_length(ctx->recv_message);
+  return (gpr_intptr)grpc_byte_buffer_length(ctx->recv_message);
 }
 
 /*