FindCommand and GlobalInvoke

why is FindCommand returning null, while GlobalInvoke works?? (the command I'd like to find is none of my commands but from another extension or Visual Studio itself... it works, when I try to find one of my commands... but that's non documented behaviour)

In my callback handler for a menu command, I have this:

var s = package.GetServiceAsync((typeof(IMenuCommandService)));
s.Wait();
OleMenuCommandService commandService = s.Result as OleMenuCommandService;
CommandID c = new CommandID(Guid.Parse("<a guid>"), 234);

MenuCommand mc = commandService.FindCommand(c);
// mc.Invoke(); -> cannot do that, because it's null

commandService.GlobalInvoke(c); // works