Collecting responses with PsychToolbox

Hello,
I am trying to collect keyboard responses with PsychToolbox using KbCheck, but when I run the code the screen freezes and refuses to close. This script is intended to advance ONLY after keyboard input 'y' or 'n' (yes/no). I also want to collect which key response was made and the response time. Here is the code I am using to collect responses:

Screen('DrawText', window, 'XXXXX') %placeholder for of screen that is drawn
Screen('Flip', window)

acceptedKeys = [KbName('y'), KbName('n')];
responded = 0;
while responded == 0
[tmp,KeyTime,KeyCode] = KbCheck;
if KeyCode(acceptedKeys)
strResponse = KeyCode
responded = 1;
end
% time between iterations of KbCheck loop
WaitSecs(0.001);
end

Screen('CloseAll');

Do you have any advice on why the screen is freezing? Thank you!

Hailey
Student, University of Oxford