#6 Command 21 encountered

[Anonymous]

I am trying to download a DRM-encrypted MMS stream from Channel 4 in the UK. At the time of writing, the URL was mms://a43.v15478f.c15478.g.vm.akamaistream.net/7/43/15478/v0001//origin.channel4.com/4od_stream/encrypted/PC_LOWRES_STREAM/45624/1/5005901/CH4_01_02_05_45624001001001_001.wmv
The download fails. Running wgetpro with "-d" reveals that mms_get_mpkt_buf (in mms.c) encounters an "unknown command 0x21". Does anyone know how to deal with this command?