[Therion] Generated PDF compliant to spec?

I tried converting my PDF map to a JPG using Ghostscript yesterday. It seemed to convert properly, though I saw a handful of warnings that there was an "Unknown operator: 'pop'" in the source PDF. Is Therion producing something which isn't to spec, or producing a newer version PDF that Ghostscript isn't prepared for? I was using Therion 5.3 Thanks, Carl