Please tell me why this isn't working.

Function SumInvoice(myC As Range) As Long
Dim Est As Long
Dim Fore As Long
Dim Prev As Long

Est = myC.Offset(0, 3).Value + Cells("K134")
Fore = myC.Offset(0, 4).Value + Cells("K135")
Prev = myC.Offset(0, 5).Value + Cells("K136")

If myC.Cells.Text < "Z*" Then
SumInvoice = 0

ElseIf Est 0 Or Fore 0 Then
SumInvoice = (Est + Fore) - Prev

Else: SumInvoice = 0

End If
End Function