SimpleFlow produces indeterminate values

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Hi, I am trying to track particular pixels using the SimpleFlow algorithm;

calcOpticalFlowSF(Mat& from, Mat& to, Mat& flow, int layers, int averaging_block_size, int max_flow)

The resulting flow matrice contains the value -1.#IND in most positions, this value indicates the calculation of sqrt(-1). In the demo in the OpenCV sample Simpleflow_demo.cpp, a test on the flow matrice values is performed using the function cvIsNaN(), it means this strange value is expected to appear. Is it a bug? how can I track the pixels when I get these values in the flow matrice? I use Opencv 2.4.6.

Thanks