Error : One of the identified items was in an invalid format.

 I keep gettig the error System.FormatException: One of the identified items was in an invalid format.
When trying to to create a url string. 
Can anyone help?
Thankyou 
 
code:

Dim PageNumber As Integer = 1 
nextbutton.NavigateUrl = "services.aspx?q=" & postedq & "&type=" & postedtype & "&nextrow=" + (PageNumber + 1)
 
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