System tiff2pdf fails

When executing the following command from the dialplan it fails…
exten => in_fax,2,system(tiff2pdf -p letter ${FAXFILE} -o ${FAXFILE}.pdf)

output:
Jun 21 17:34:38 VERBOSE[8429] logger.c: – Executing System(“Zap/1-1”, “tiff2pdf -p letter /var/spool/asterisk/fax/1150911243.52.tif -o /var/spool/asterisk/fax/1150911243.52.tif.pdf”) in new stack

Jun 21 17:34:38 VERBOSE[8429] logger.c: – Executing NoOp(“Zap/1-1”, “tiffresponse: APPERROR”) in new stack

If I copy this line and run it manually on the machine, it executes without a problem. Any ideas on why this may be happening?