knn matcher fails

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i am extracting ORB features from 1500 images i.e. 200 features from every training image. I store features in yml files and load them in one matrix. When I try to apply knn search during the image matching i am getting "Assertion Error". When I try to use small number of training images then it works perfectly fine. This shows that knn is failing with large features such as 2 M . How to resolve this issue?