NSApplication endSheet: has no effect

Ok, this is really stumping me:

I create a modal sheet on a window as

-(IBAction) login: (id) sender {
[NSApp beginSheet:loginWindow 
   modalForWindow:window 
    modalDelegate:nil 
   didEndSelector:nil 
      contextInfo:nil];
}

and try to remove it with

-(IBAction) loginWindowCancelPressed:   (id) sender {
debugLog(@"cancel");
[NSApp endSheet:loginWindow];
}

but it remains.

I've checked all of the obvious things like IB connections and ensuring that window and loginWindow are present and in the normal state.

So, the question is: Am I missing something?