error using nil=true

Hello,

I have a soap Request that contains a line like this:
<col:_userinfo i:nil="true" />

However, when I validate the Request, I get the following error message:
line 74: Attribute not allowed (no wildcards allowed): nil@i in element _userinfo@http://schemas.datacontract.prg/2004/...

Shouldn't that line work?

I'm quite new to soap and soapUI, so I apologize if I'm asking a silly question.

Thanks.

(By the way, I'm using soapUI 4.0.0 build dist-123-2011-06-14.)