Blank video (swscaler error) on ProRes 422 HQ input files

The ffmpeg2theora command line client on OS X fails when using ProRes 422 (HQ) video files as input. ffmpeg parses the incoming file and interprets and reports all its streams properly, but I get an swscaler error (at different hex addresses) during the actual encode process (regardless of whether I'm scaling the video or not).

Command line is as follows:

ffmpeg2theora -V 2850 --two-pass --optimize -x 920 -y 518 --aspect 16:9 -K 120 -A 128 -c 2 -H 48000 -o Output.ogv -i Input.mov

Where Input.mov is a QuickTime ProRes 422 (HQ) file output from Final Cut Pro. The encoder reports the following:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'TESVSkyrim_FullTrailer_US.mov':
  Metadata:
    major_brand : qt
    minor_version : 537199360
    compatible_brands: qt
    creation_time : 2012-02-02 02:17:14
  Duration: 00:02:53.70, start: 0.000000, bitrate: 183004 kb/s
    Stream #0.0(eng): Video: Apple ProRes 422 (HQ), 1280x720, 180665 kb/s, PAR 1:1 DAR 16:9, 59.94 fps, 59.94 tbr, 5994 tbn, 5994 tbc
    Metadata:
      creation_time : 2012-02-02 02:17:14
    Stream #0.1(eng): Audio: pcm_s24le, 48000 Hz, 2 channels, s32, 2304 kb/s
    Metadata:
      creation_time : 2012-02-02 02:17:14
    Stream #0.2(eng): Data: tmcd / 0x64636D74
    Metadata:
      creation_time : 2012-02-02 02:20:42
  Pixel Aspect Ratio: 1.00/1 Frame Aspect Ratio: 1.78/1

Then I get the following error:

[swscaler @ 0x1894600] (null) is not supported as input pixel format

The encoder then starts the first pass, but reports the error again.

Scanning first pass pos: 0:02:53.70 ET: 00:00:00 [swscaler @ 0x1892c00] (null) is not supported as input pixel format

And the encoder will finish the encode, but the result is a blank file (all black) with audio and no picture.