opencv + python：findcontours时断言失败

opencv+python: Assertion failure when findcontours

我正在做一个图像处理项目。

我使用的是opencv 2.4.11+python 2.7

代码很简单

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def preprocessing(src):
src = cv2.resize(src, (0,0), fx=6, fy=6)
kernel = np.ones((5,5), np.uint8)

dilate = cv2.dilate(src, kernel, iterations = 1)
erode = cv2.erode(dilate, kernel, iterations = 1)

cv2.imshow("dilate", dilate)
cv2.imshow("erode", erode)

th, thresh = cv2.threshold(erode, 200, 255, cv2.THRESH_BINARY)
cv2.imshow("thresh", thresh)

contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
cv2.drawContours(src,contours,-1,(0,255,0),3)

cv2.imshow("src", src)
cv2.waitKey()

print"==== start image processing ====="
im2 = cv2.imread("training1.png",cv2.CV_LOAD_IMAGE_GRAYSCALE)
preprocessing(im2)

但是findcontours返回断言失败

输出是

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OpenCV Error: Assertion failed (step[dims-1] == (size_t)CV_ELEM_SIZE(flags)) in create, file /tmp/opencv20160107-29960-t5glvv/opencv-2.4.12/modules/core/src/matrix.cpp, line 236
Traceback (most recent call last):
File"process.py", line 55, in <module>
preprocessing(im2)
File"process.py", line 44, in preprocessing
contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)
cv2.error: /tmp/opencv20160107-29960-t5glvv/opencv-2.4.12/modules/core/src/matrix.cpp:236: error: (-215) step[dims-1] == (size_t)CV_ELEM_SIZE(flags) in function create

有人能帮忙吗？非常感谢你