Error while detecting Face and Eye using OpenCV (Haar Cascade Object Detection)

Question

I am trying to detect Face and Eye in OpenCV with Python. I am using below code.

face_cascade =cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')

cap = cv2.VideoCapture(1)

while True:
ret,frame =cap.read()
gray = cv2.cvtColor(frame,cv2.COLOR_BGR2GRAY)
faces = face_cascade.detectMultiScale(gray,1.3,5)
for(x,y,h,w) in faces:
    cv2.rectangle(frame ,(x,y),(x+w,y+h),(255,0,0),2)
    roi_gray = gray[y:y+h,x:x+w]
    roi_color = img[y:y+h,x:x+w]
    eyes = eye_cascade.detectMultiScale(roi_gray)
    for(ex,ey,ew,eh) in eyes:
        cv2.rectangle(roi_color,(ex,ey),(ex+ew,ey+eh),(0,255,0),2)

cv2.imshow('img',frame)
k = cv2.waitKey(30) & 0xFF
if k == 27:
    break
cap.release()
cv2.destroyAllWindows()

I am getting the following error:

Traceback (most recent call last):
File "C:/Users/Keval Rathod/Documents/opencv/img/markertracking.py", line      11, in <module>
gray = cv2.cvtColor(frame,cv2.COLOR_BGR2GRAY)
error: ..\..\..\..\opencv\modules\imgproc\src\color.cpp:3739: error: (-215) scn == 3 || scn == 4 in function cv::cvtColor