diff --git a/lib/multipartform.js b/lib/multipartform.js
index 7196724..475f77d 100644
--- a/lib/multipartform.js
+++ b/lib/multipartform.js
@@ -128,7 +128,7 @@ Part.prototype = {
   	      }); 
   	    })(); // reader() 
   	  });
-     } else if (this.value instanceof Data) {
+     } else if (this.value.data) {
   	  stream.write(this.value.data);
   	  stream.write("\r\n");
   	  callback();