ASP Len() function returns Type Mismatch error

Function isValidPACSSession() ... Dim sessionID : sessionID = Request.QueryString("forSessionID") isValidPACSSession = SessionID2PACSUserID(sessionID) ... End Function

Function SessionID2PACSUserID(sSessionID) if (Not IsNull(sSessionID) AND Len(sSessionID) > 0) then ...it fails here at the Len function. else ... end if End Function