Using custom headers in makeWebRequest

I'm trying to append custom headers when calling makeWebRequest, for example:

Comm.makeWebRequest(
url,
{
"token" => token
},
{ :method => Comm.HTTP_REQUEST_METHOD_POST,
:headers => {
"X-ApiVersion" => api
}
},
method(:onReceive)
);

But I always get a -200 error (INVALID_HTTP_HEADER_FIELDS_IN_REQUEST), so I suspect that this is not allowed, am I right?