Problem opening FLV file

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Hello. Years ago, I developed some code for processing MKV files. Now I need it to work with FLV files too. However, the same code which works with MKV:

new VideoCapture(file);

fails with FLV:

warning: Could not find codec parameters (../../modules/highgui/src/cap_ffmpeg_impl.hpp:556)

I'm using OpenCV 2.4.13 in VS2017 / Windows 10. FLV file plays OK in VLC and MPC. It contains only video (FSV1 codec) and can be transcoded with ffmpeg without issues.

Thanks.