Error when getting the attribute value through soapUI

I have a webservice which uses the the attribute @id at a List. It should like this ‘XXX’ ,I make a document as response , and give a documentlist in it , corresponding the above country list. so when I test through i got the the exception " webM:exception
webM:classNamejava.lang.NullPointerException</webM:className>
<webM:message xml:lang=""/>
</webM:exception>"
has anyone here know the issues? and can you help with it .
I really need your help , Thanks In advance.