For/ Next loop not working

[Osprey2]

I am trying to clear old entries in TextBoxes using this code

 With UserForm1
   For a = 1 To 6
   .TextBox & "a".Value=""
   Next a
   End With

This fails with the & highlighted and a "Syntax error"
Why is this wrong?
John

[ByteMarks]

"a" is the letter a
Try:

Dim a As Integer
With UserForm1
    For a = 1 To 6
        .Controls("TextBox" & a).Value = ""
    Next a
End With