handling blank input box

If I do this:

salesgoal = Application.InputBox("What is the sales goal for the month?", Title:="Holidays", Type:=1)

If the user clicks "OK" without entering anything, it produces an error. I tried handling it with

If salesgoal = vbNullString Then
    Exit Sub
End If

but it doesn't get there, excel throws an error before it runs that "if null" line.

Is there a way to fix this?

Thanks,
Dave