ofxFaceTracker classifier class randomly bug

working with ofxFaceTracker Classifier to detect smile, I notice that I have a bug that randomly comes up.

the problem seems to be on this file, line 50.
ofxfacetracker/scr/ExpressionClassifier

double v = norm(cur, expressions[i].getExample(j));

it says “Thread 1 : signal SIGABRT”
and console displays

   OpenCV Error: Assertion failed (src1.size == src2.size && src1.type() == src2.type()) in norm, file   /Users/theo/Downloads/OpenCV-2.3.1/modules/core/src/stat.cpp, line 1128
libc++abi.dylib: terminating with uncaught exception of type cv::Exception: /Users/theo/Downloads/OpenCV-2.3.1/modules/core/src/stat.cpp:1128: error: (-215) src1.size == src2.size && src1.type() == src2.type() in function norm

Since sometimes it work sometimes it dont, I think using OfxSmile looks more stable.
just wanted to report that bug.