Custom SqlProfileTableProvider and UserID

I implemented the SqlProfileTableProvider and as a primary key it uses the UserID.
In my web.config where we list the properties:
<add name="CompanyID" type="int" defaultValue="[null]" customProviderData="CompanyID;int"/> <add name="EmployeeID" type="int" defaultValue="[null]" customProviderData="EmployeeID;int"/> <add name="UserID" type="string" defaultValue="[null]" customProviderData="UserID;nvarchar"/> <add name="BusinessName" type="string" defaultValue="[null]" customProviderData="BusinessName;nvarchar"/>
I was trying to add the UserID as a name that could be called using Profile.UserID eg. Dim userid = Profile.UserID
When I try to do this I get an error: Exception Detail ...