[Python] OpenCV show err.message at camera launch

I would like to launch a usb-camera at my raspberry pi, but when I try to start python code (opencv), it shows this message and stop a launch

ASSERT: "false" in file qasciikey.cpp, line 501
Aborted


Can someone please explain me, what error is this? It doesnt work even with another codes as well... But camera works fine, when I open it in programs like Camorama webcam viewer. I read that this part makes a problem

cv2.waitKey(20)


So it makes in another code (when i uncomment it, it launch the code but dont show camera output) But in this code, even if I uncomment it, the code themself doesn't work: show upper error message

Here is the code


import numpy as np
import cv2

cap = cv2.VideoCapture(0)

while(True):
# Capture frame-by-frame
ret, frame = cap.read()

# Our operations on the frame come here
gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

# Display the resulting frame
cv2.imshow('frame',frame)
cv2.imshow('gray',gray)
if cv2.waitKey(20) & 0xFF == ord('q'): # HERE
break

# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

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