(-215) scaleFactor > 1 && image.depth() == CV_8U in function detectMultiScale

edit

I run the code below taken from documentation:

 import numpy as np
 import cv2

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

 img = cv2.imread('sachin.jpg')
 gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)


 faces = face_cascade.detectMultiScale(gray, 1.3, 5)
 for (x,y,w,h) in faces:
   cv2.rectangle(img,(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',img)
cv2.waitKey(0)

cv2.destroyAllWindows()

But I got this error message:

Traceback (most recent call last):
  File "testa.py", line 10, in <module>
    faces=face_cascade.detectMultiScale(gray,1,3,5)
cv2.error: /home/nakkini/opencv/opencv-2.4.10/modules/objdetect/src/cascadedetect.cpp:1081: error: (-215) scaleFactor > 1 && image.depth() == CV_8U in function detectMultiScale

How can I resolve this problem ?

Thank you very much in advance.