WebClient illegal characters in path

I am trying to use a WebClient to get the content of another webform in my project.   I am using a WebClient because I want to do this asynchronously, so if there is a better way to do that, I am open to it.
My webclient, however, is erroring with the "Illegal characters in path" error.   Looking at it in the debugger, I see that this is the URI string I am using:  
"PortfolioDetail.aspx?CCSG=True&Open=False&Plcmt=2008-Aug "
I don't see any illegal characters in it... what am I doing wrong?
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