[GWT with Internet Explorer ]: FormPanel SubmitCompleteHanlder event.getResults returns null

Question

I am trying to do a POST operation which uploads a file.

I am using GWT 1.8 FormPanel to submit the file.

When I upload the file, server validates the file and returns response as String.

If the response code is 200, submitCompleteEvent.getResults() have response text.

For example,

<pre style="word-wrap: break-word; white-space: pre-wrap;">
     {"message":"Invalid file!!"}
</pre>

But when the response code is changed to 400 in the server, submitCompleteEvent.getResults() returns null.

This is happening only in Internet Explorer. I am using Internet Explorer 11. For other browser, it is working fine.