null pointer exception on Java HshMap after it is pulled from another HashMap

Hi,

I'm receiving a very bizarre null pointer error from a Java HashMap after I pull it out of another HashMap.          I have a function (part of which is shown below).  In it I have a while loop that loops through and puts Answer objects into a HashMap called am.  After this happens, I've run a quick test to output the size of the am HashMap.  This code works.  

However, after the while loop I insert the am HashMap into another HashMap called fa.  If, at this point, I get the HashMap back out of the fa HashMap and store it in am2, and try to get the size of am2, I get a java.lang.NullPointerException.  I can't figure out why this is happening.  I've also put in some test code to loop through the keys in the fa HashMap and print out the contents.  It does this just fine and shows all of the data for everything that should be in am2.

If anyone has any ideas why it would stop working after I pull the am2 HashMap out of the fa HashMap, that would be great!

Thanks,
Courtenay

// Declare AnswerMap
        HashMap am = new HashMap();

        // Declare FormAnswers
        HashMap fa = new HashMap();


 java.util.Iterator i = questions.iterator();
while (i.hasNext()){      
 ............
            // Create an Answer object and insert the fid, ftid, qtid and
            Answer a = new Answer(fid_string, ftid_string, qtid_string, adv);

            // add the Answer object to the AnswerMap
            // (declared before the while loop)
            am.put(qtid_string, a);

      // this output statement works fine
       System.out.println("FormProcessingServlet.java: storeSubmittedFormPageData(): am.size() = " + am.size());

                        // test to make sure the am has the a
                        Answer a2 = (Answer) am.get(qtid_string);

   
       } //end while


        // Add the AnswerMap to FormAnswers
       // fa declared before while loop
        fa.put(ftid_string, am);

             // test loop through fa HashMap.  This correctly shows all the data that should be available in am2
            Set fa_keys = fa.keySet();
            System.out.println("FormProcessingServlet.java: storeSubmittedFormPageData(): fa_keys.size() = " + fa_keys.size());
            java.util.Iterator i2 = fa_keys.iterator();
            while (i2.hasNext()){
                        String key = (String) i2.next();
                        System.out.println("FormProcessingServlet.java: storeSubmittedFormPageData(): fa_key/ftid = " + key);
                        System.out.println("FormProcessingServlet.java: storeSubmittedFormPageData(): am = " + fa.get(key));

            }


            HashMap am2 = (HashMap) fa.get("ftid_string");  // this line has no errors

       System.out.println("FormProcessingServlet.java: storeSubmittedFormPageData(): am2.size() = " + am2.size()); // this line causes the exception.