Why can I get contours from a 'thresh' image but not from a 'mask' image when using VideoCapture?

edit

import cv2
import numpy as np

cap = cv2.VideoCapture(0)

while (1):
    _, frame = cap.read()

    gray = cv2.cvtColor(frame,cv2.COLOR_RGB2GRAY)
    ret, thresh = cv2.threshold(gray,50,255,0)

    # Convert BGR to HSV
    hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)

    # define range of blue color in HSV
    lower_blue = np.array([110, 175, 50])
    upper_blue = np.array([130, 255, 255])

    # Threshold the HSV image to get only blue colors
    mask = cv2.inRange(hsv, lower_blue, upper_blue)

    _, contours, hierarchy = cv2.findContours(thresh, 1, 2)
    cnt = contours[0]

    #_, contours, hierarchy = cv2.findContours(mask, 1, 2)
    #cnt = contours[0]
    ## IndexError: list index out of range


    cv2.imshow('frame', frame)
    cv2.imshow('mask', mask)
    cv2.imshow('thresh',thresh)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

cv2.destroyAllWindows()

I've been able to determine that both 'thresh' and 'mask' are matrices each of 480 rows and 640 columns and each cell contains either a '0' or '255'. Also, 'thresh' and 'mask' are of type 'numpy.ndarray'.

So why is this valid:

_, contours, hierarchy = cv2.findContours(thresh, 1, 2)
cnt = contours[0]

But this is not:

_, contours, hierarchy = cv2.findContours(mask, 1, 2)
cnt = contours[0]
# cnt = contours[0]
# IndexError: list index out of range