makeWebRequest doesn't accept valid JSON?

I'm making a web request using this:

```
Communications.makeWebRequest(
...,
...,
{
:method => Communications.HTTP_REQUEST_METHOD_POST,
:headers => {
"Content-Type" => Communications.REQUEST_CONTENT_TYPE_JSON
},
:responseType => Communications.HTTP_RESPONSE_CONTENT_TYPE_JSON
},
method(:loginResponseCallback)
);
```

In "View HTTP Traffic" I can see the response has this header:

```
Content-Type: application/json\r\n
```

and after the header it has this contents:

```
"..."
```

However in my callback I get `null` as the data, and -400 as the code.

Any ideas what the problem may be? Does it expect to see a JSON object maybe? If it does, is there a way to work around this problem?