VBA Getting Runtime Error 94 Invalid Use of NULL

Being new to VBA, common errors like Run Time Error 94 can be hard to figure out. In this example, I was able to resolve the error by NOT declaring the variable CatVal as a String


Private Sub CommandButton3_Click()

' COMMENTING OUT THE NEXT LINE TOOK CARE OF THE RUN-TIME 94 INVALID USE OF 'NULLS
'Dim Cat2Val As String

With Me.Categories2
    Cat2Val = .Value
If IsNull(Cat2Val) Then
    MsgBox ("Select a value for Group 2")
Else:
    PubCat2Val = Cat2Val

End If
End With

End Sub