Some keystrokes are resticted to the OS. Don't know about this one in particular but Alt-F4 is the default to close a program and it can't be bound. I opened a ticket and was told that it is restricted. I'm sure there are others. If you Goggle ctrl+alt+1, you'll find that other progams can't bind it either.
Here is an article about never using ctrl-alt for binding because it's used to generate special characters. It's also used by the Windows OS as a shortcut key for menu items. It seems to be a bad practice to use it in keybinding. My guess is you have it mapped at the OS level to a menu command or it's generating a special character.