mediafilesegmenter fails to segment a file due to track containing contains more than one valid time mapping

When I try to run mediafilesegmenter with the following

mediafilesegmenter -iso-fragmented -z frame_index.m3u8 \
                  -f 720p -i index_720p \
                  -k key.bin -stream_encrypt -P
                  -K <url> \
                  -B seg_720p -t 6 source.mp4

I get the following output

ISO fragmented mode, forcing segments to start with I-Frame                                                                                                                                                                     
Processing file source.mp4                                                                                                                             
track 2 of source.mp4 contains more than one valid time mapping                                                                                        
Unable to find any valid tracks to segment.                                                                                                                                                                                     
Segmenting failed (-15650).   

What specifically should I be looking for in my MP4 that would be causing this?