Replacing VbTab with Underscore in ActiveSheet

[Rasm]

I have the following code - But I get runtime error 13 - type mismatch.

What I am trying to do is replace any VbTab character in any cell with an underscore in my ActivSheet. The code can be completely different from my approach - it does not work anyway.

Private Sub ReplaceVbTabWithUnderscore()
    'Makes sure that the individual cells dont contain a VbTab
    'VbTab is used as delimiter for keys in the TreeView
    'Any VbTab in a cell is replaced with an _ (underscore)
    Dim RangeVal As String
    With ActiveSheet.UsedRange
        LastRow = .Rows(.Rows.Count).Row
    End With
    With ActiveSheet.UsedRange
        ColLast = .Columns(.Columns.Count).Column
    End With
    RangeVal = "A1:" & Split(Cells(1, ColLast).Address, "$")(1) & LastRow
    ActiveSheet.Range(RangeVal) = Replace(ActiveSheet.Range(RangeVal), vbTab, "_")
End Sub