TypeError when handling errors in gstlal.dagparts.which

When executing gstlal_fake_frames_pipe on a system that doesn't have gstlal-ugly installed, I get the following TypeError:

  File "/Users/duncan/opt/miniconda3/conda-bld/gstlal_1549462772035/bin/gstlal_fake_frames_pipe", line 78, in gstlal_median_psd_job
    def __init__(self, group, user, executable=dagparts.which('gstlal_median_of_psds'), tag_base='gstlal_median_of_psds'):
  File "/Users/duncan/opt/miniconda3/conda-bld/gstlal_1549462772035/lib/python2.7/site-packages/gstlal/dagparts.py", line 65, in which
    print >>sys.stderr, "ERROR: could not find %s in your path, have you built the proper software and sourced the proper environment scripts?" % (prog,prog)
TypeError: not all arguments converted during string formatting