http request set header exception

Hi,

I have problems setting a header in my http request, I get an exception code (nullpointer). I'm 1000% sure the header has a value.

'set URL request
Dim urlRequest As HttpRequest
'request settings
If httpHeader <> "" Then
urlRequest.SetHeader (httpHeader,token)
Else
End If

Are there limitations on what the header can contain because I use my own custom headers.