(-13) Matrix error in Eigenfaces model (Face Recognizer)

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Hi guys, I've been experiencing this error on Eigenfaces.cpp:

OpenCV Error: Image step is wrong (The matrix is not continuous, thus its number of rows can not be changed) in reshape, file terminate called after trowing an instance of 'cv::Exception'
what (): /home/pi/opencv-2.4.8/modules/core/src/matrix.cpp: 802: error: (-13) The matrix is not continuous, thus its number of rows can not be changed in function reshape

I downloaded a pack of 400 faces, but I don't know what this error is about. It is about the faces, the filters or algorythm problem?

It is called on the 'reshape' method

  // perform the PCA
PCA pca(data, Mat(), CV_PCA_DATA_AS_ROW, _num_components);
 // copy the PCA results
_mean = pca.mean.reshape(1,1); // store the mean vector

Edit: I'm using OpenCv 2.4.8