Solved must be of the type boolean, none returned

what does this mean?

Code:

 [Server thread/CRITICAL]: Unhandled exception executing command 'quit' in quit: Return value of hi\Main::onCommand() must be of the type boolean, none returned
[13:54:52] [Server thread/CRITICAL]: TypeError: "Return value of hi\Main::onCommand() must be of the type boolean, none returned" (EXCEPTION) in "hi/src/hi/Main" at line 53

 

well i just add a return true; and it stopped giving me error.