Looking at that particular file, flamethrower.peg_pc doesn't have any textures inside of it, but it still did work successfully for me and create a .desc file. If you look at your screenshot you'll even see that both files are only 1k. Try extracting an actual texture this time like from p_flamethrower.peg_pc (and naturally it's corresponding g_peg_pc file) and see if you still get an error.