"Proxy crashed" on MacPorts OS X

I have installed Kdenlive via MacPorts on a MacBook Pro running OS X Snow Leopard. My goal is to edit some h264 1080p (actually 1088p, my camera likes to be nonstandard) video that has just not been tractable in iMovie or other closed apps.

What I'm encountering is a problem with proxy clips, namely, I always get a "Proxy crashed" message when trying to generate them. The thing that's confusing me is that I cannot understand *why* this is crashing. According to my project's settings, the proxy clips are generated by:

ffmpeg -i [movie name].MP4 -f mpegts -acodec copy -ac 2 -ab 128k -ar 48000 -vcodec mpeg2video -g 5 -deinterlace -s 480x270 -vb 400k [movie name].ts

When I run this from the command line, it works just fine, and I can take the clip that is output and put it into a Kdenlive project. What is not working is calling this same conversion from within Kdenlive, having it automatically organize my proxy files, and then being able to directly command rendering of the full-res files. Help?