Run-time error '13' with drop-down list

Hello,

I succeed to have if A1 not empty then B1 = A1 but if at the same time B1 = not empty, then do not replace B1 by A1 :

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Range("A1") And Range("B1") = "" Then
Range("B1") = [A1]
End If
End Sub

The problem is that it doesn't work when cells A1 and B1 include drop-down lists with letter and number : D4, B3, F2... (but it works if only numbers : 22, 56...)

I have : "Run-time error '13' type mismatch"

Is-it possible to fix it ?

Thanks a lot for the help