I was not getting an error at console when trying to use the software. I captured this with tcpdump. From what I have gathered it is failing on comboot.c on a size check.
302 /* Sanity check for filesize */
303 if( image->len >= 0xFF00 ) {
304 DBGC( image, "COMBOOT %p: image too large\n",
305 image );
306 return -ENOEXEC;
307 }
Not sure where the DOS part came from, tcpdump isn't always perfect.