Spring security Custom Filter: cannot read FROM POST parameter

I am trying to read a form-post parameter with the following code:

String hohumId = request.getParameter("HOHUM_ID");

if (hohumId == null) {
throw new
    PreAuthenticatedCredentialsNotFoundException("ID not found in request");
}

This is inside a Spring-Security Custom Filter, the result is always null. I have used Tamper Data in FireFo开发者_C百科x and the form parameter is in the 'Post Parameter List'.

It is as though the form data is being mysteriously stripped away. My question is very similar to the following question. Couldn't find a solution though.

POST parameters mysteriously eaten after using spring security & an ntlm filter