Mismatch error 13 breaking command button

OK not quite so dramatic but darn frustrating....

On a numer of spreadsheets I get a "mismatch error 13" and as a result I am resigned to using two buttons for what should be a single function (and button) as the second function I want to run requires the first to be carried out first....

The mismatch error does not stop the first function working, but does stop the code in its track and pops up the error mismatch message.

I have tried ignore error commands but whilst they stop the pop up the code won't move on to the next string of code successfully. Has anybody had this type of error before and know which ignore error command will not only stop the pop up message being shown but also allow the two functions to run concurrently without stopping?

If there's any takers on this, the error points at the "TestVal = TestCell.Value" line....

Option Explicit

Private Sub CommandButton1_Click()

' Sub ReplaceF()

Dim RefRange As Range, _
TestCell As Range, _
LastGRow As Long, _
TestColumn As Long, _
TestVal As Long

LastGRow = Cells(Rows.Count, "G").End(xlUp).Row
Set RefRange = Range("G4:G" & LastGRow)

'increment down refrange
For Each TestCell In RefRange
TestCell.Select
If TestCell.Value = "" Then
TestVal = 0
Else
TestVal = TestCell.Value
End If

'check: if col-f cell has same value quit
If TestVal = Cells(TestCell.Row, "F").Value Then
Exit Sub
End If

'if testval is greater than zero replace col-f val with g val.
If TestVal <> 0 Then
Cells(TestCell.Row, "B").Value = Cells(TestCell.Row, "C").Value
Cells(TestCell.Row, "C").Value = Cells(TestCell.Row, "D").Value
Cells(TestCell.Row, "D").Value = Cells(TestCell.Row, "E").Value
Cells(TestCell.Row, "E").Value = Cells(TestCell.Row, "F").Value
Cells(TestCell.Row, "F").Value = TestVal
End If

Next TestCell

End Sub