Hi,I'm new to madmom and I'm having a problem trying to use LogarithmicFilteredSpectrogram with a stereo wav file. I've tried loading the file using load_wav_file and specifying num_channels = 1, but when I pass the output (first returned value) to LogarithmicFilteredSpectrogram unsupported operand types for :/ 'float' and 'NoneType'. Any suggestions on how to proceed?