error when call JSON response

Hello, I don't know this is bug or not. But ...

I have Controller A with method like :

Code:

public function myjson() {
     ....
     ....

     return $this->respond($json); // output JSON    
}

when call : http://localhost:8080/a/myjson
showing the output, JSON

but the problem is, when call by other controller show Error like
"Call to a member function setContentType() on null" ...
   
____
when code changed to

Code:

public function myjson() {
     ....
     ....

     return json_encode($json); // output JSON    
}

no error