Excel 2010 :: Macro Save File As XLXS But Not XLSM

I am using the macro below to save a file. It works with the extension .xlsx but not xlsm. I get a message that I can't use this extension for this file type. I am missing something fundamental.

Using Excel 2010
Sub testsave()'
Dim a As String
Dim b As String
Dim c As String
Dim d As String
b = "Myyfile"
c = b & ".xlsm"
a = ThisWorkbook.Path
d = a & "" & c
ActiveWorkbook.SaveAs Filename:=d
End Sub

FYI - there are no macros in the file I am trying to save.