Validating A Variable In VBA

It's the last road-block left to making this function work. The Problem is in the If...Then statement. If A is true the formula works, but is it is not, then it returns a value. The Expression

IBGLink = (E & CustomDate & EndOfLink)
works on its own, so there is an issue with

If Application.WorksheetFunction.IsError(A) Then
Does anyone know an alternative way of validating a variable or a trick of sort that can be used here. A to E and CustoDate and CustDatePN are declared as global variables.


Function IBGLink(IBG_URL As String, FrstDate As Date, ScndDate As Date, Optional EndOfLink As String)
A = Mid(IBG_URL, Application.WorksheetFunction.Find("/200", IBG_URL), 8)
B = Left(IBG_URL, Application.WorksheetFunction.Find("/200", IBG_URL))
C = Right(IBG_URL, Len(IBG_URL) - (Len(A) + Len(B)))
D = Left(C, Application.WorksheetFunction.Find(".200", C))
E = Left(IBG_URL, Application.WorksheetFunction.Find(".200", IBG_URL))
CustomDate = Format(FrstDate, "yyyymmdd")
CustDatePN = Format(ScndDate, "yyyymmdd")
If Application.WorksheetFunction.IsError(A) Then
IBGLink = (E & CustomDate & EndOfLink)
Else: IBGLink = (B & CustomDate & D & CustDatePN & EndOfLink)
End If ................