AppleScript : CMD + S I can't manage to keystroke Command + S on an app, it's seems to trigger "Save AS" instead of "Save"

 tell application "TextMate 2"
    activate

    tell application "System Events"
        keystroke "S" using {command down} 
    end tell

    --
end tell


I don't have "shift" in my keystroke so I don't know why I can just save my documents without any extra windows.

Any idea of what I'm doing wrong?
Use a Lower Case s.

keystroke "s" using {command down}