[Python] OpenCV error: (-215:Assertion failed) size.width>0 && size.height>0 in function...

I am new to OpenCV and am trying to make a live face tracker on my computer. I seem to be having an issue with my video input (is it not getting any?). I'm on a mac laptop and am trying to use my built-in webcam.

Error:

cv2.error: OpenCV(4.3.0) /Users/travis/build/skvark/opencv-
python/opencv/modules/highgui/src/window.cpp:376: error: (-215:Assertion failed) size.width>0
&& size.height>0 in function 'imshow'


Code:

import numpy as np
import cv2

cap = cv2.VideoCapture(0)

while True:
#capture frame by frame
ret, img = cap.read()
cv2.imshow('test', img)


cv2.waitKey(0) & 0xFF
cv2.destroyAllWindows()
cap.release()


I tried to change cv2.VideoCapture(0) to cv2.VideoCapture(1), but that resulted in this error:

You might be loading two sets of Qt binaries into the same process. Check that all plugins
are compiled against the right Qt binaries. Export DYLD_PRINT_LIBRARIES=1 and check that
only one set of binaries are being loaded.
QObject::moveToThread: Current thread (0x7ff06a82c5d0) is not the object's thread
(0x7ff06a876c30).
Cannot move to target thread (0x7ff06a82c5d0)

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