VBA Inserts "NM" instead of 0""

Try

Sub InsertIfError()
'
' InsertIfError Macro
'
' Keyboard Shortcut: Ctrl+Shift+E
'
    Dim c           As Range
    Dim f           As String

    For Each c In Selection
        f = Mid(c.Formula, 2)
        c.Formula = "=iferror(" & f & ", 0 )"
    Next c

End Sub