Chrome extension commands (keyboard shortcuts) not working

Any idea how I can get Chrome extension keyboard shortcuts working on a Mac? I have this in my manifest.json

"commands": {
  "trigger_me": {
    "suggested_key": {
      "default": "Ctrl+E"
    },
    "description": "Trigger test"
  }
}

When I look in the Chrome Extensions tab under Keyboard shortcuts I can see the 'Trigger test' entry but the actual shortcut is not set.

How can I set a default shortcut via manifest.json?