OpenCV Error: Assertion failed ((scn == 3 || scn == 4) && (depth == CV_8U || depth == CV_32F))

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Currently i am trying to track objects by color. I've run the documentation code, however, it gives the following sentences. The documentation code is here

OpenCV Error: Assertion failed ((scn == 3 || scn == 4) && (depth == CV_8U || depth == CV_32F)) in cvtColor, file /home/guerrero/opencv2.4.9/opencv-2.4.9/modules/imgproc/src/color.cpp, line 3959 terminate called after throwing an instance of 'cv::Exception' what(): /home/guerrero/opencv2.4.9/opencv-2.4.9/modules/imgproc/src/color.cpp:3959: error: (-215) (scn == 3 || scn == 4) && (depth == CV_8U || depth == CV_32F) in function cvtColor

Actually, I've searched the meaning of the errors on the Internet, but it gives a limited explanation. Does anyone help me to modify the code a little bit, please? The documentation code is provided below. Thank you very much!

import cv2

import numpy as np

cap = cv2.VideoCapture(0)

while(1):

# Take each frame
_, frame = cap.read()

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

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

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

# Bitwise-AND mask and original image
res = cv2.bitwise_and(frame,frame, mask= mask)

cv2.imshow('frame',frame)
cv2.imshow('mask',mask)
cv2.imshow('res',res)
k = cv2.waitKey(5) & 0xFF
if k == 27:
    break

cv2.destroyAllWindows()