Open CV showing error

edit

I am using openCV with python, this code is from http://docs.opencv.org/trunk/doc/py_tutorials/py_imgproc/py_colorspaces/py_colorspaces.html#converting-colorspaces

When I run it It gives an error

hsv=cv2.cvtColor(frame,cv2.COLOR_BGR2HSV) error: ......\src\opencv\modules\imgproc\src\color.cpp:3541: error: (-215) (scn == 3 || scn == 4) && (depth == CV_8U || depth == CV_32F)

Thank You.

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_green, upper_green)

    # 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()