My VBA Code will not work when the sheet is protected?

I'm using the following VBA code...

Private Sub Worksheet_Change(ByVal Target As Range)

If Not Intersect(Target, Range("B:H")) Is Nothing Then

If Target.Count > 1 Then Exit Sub

Cells(Target.Row, "M").Value = Environ("username") & ", " & Format(Now, "yyyy/mm/dd  hh:mm:ss")

End If

End Sub

It works fine if I leave the sheet unprotected. But when I protect the sheet in order to lock specific cells, I get a Debugger message box.

Do I need to alter the VBA code to allow it to work when the sheet is protected?

Many thanks.