Why when I'm converting MKV files with "MKV To Avi With Subtitles" this message "failed pBuilder -> RenderStream ( NULL, NULL, pPin, NULL, pMux)" comes out???

Can it be avoided?

Actually, my default video compressor is ffdshow video encoder, but I only use that when the video codec of the MKV file is not H264(MPEG-4). The problem is, when I convert MKV files with a H264 video codec while my video compressor is ffdshow video encoder, the video is not converted. It takes ages and I never see it finishes. So I tried changing my video compressor to "x264vfw - H.264/MPEG-4 AVC codec" to try my luck but this message "failed pBuilder -> RenderStream ( NULL, NULL, pPin, NULL, pMux)" comes out. Any suggestions????