problem in running cvGoodFeaturesToTrack in ctypes-opencv

Question

I've been using ctypes-opencv for a while. If I run cvGoodFeaturesToTrack the way it is used in the demo, I get the following error:

OpenCV ERROR: Null pointer (NULL array pointer is passed) in function cvGetMat, cxarray.cpp(2780) Terminating the application... called from cvUnregisterType, cxpersistence.cpp(4933) Terminating the application... called from cvUnregisterType, cxpersistence.cpp(4933) Terminating the application... called from cvUnregisterType, cxpersistence.cpp(4933) Terminating the application... called from cvUnregisterType, cxpersistence.cpp(4933) Terminating the application...

Here is my code:

cornersa = []

cornersa[0] = cvGoodFeaturesToTrack(imga, eig_image, tmp_image, None, corner_count, 0.01, 10)