shortcut Ctrl+M=Mirror does not work

 From:  malvin
2397.1 
I made shortcut Ctrl+M=Mirror but for some reason it does not work for mirror it repeats the last command
Can you tell me please why.
It is not awfully important I just wonder why?
I have a whole bunch of shortcuts with Ctrl all work but Ctrl+M=Mirror
Ctrl+B=Blend
Ctrl+C=CopyClipboard
Ctrl+E=Extend
Ctrl+F=Fillet
Ctrl+L=Loft
Ctrl+M=Mirror
Ctrl+N=New
Ctrl+O=Open
Ctrl+P=Polyline
Ctrl+R=RotateAxis
Ctrl+S=Save
Ctrl+V=Paste
Ctrl+X=Cut

Thanks
Olga