NifsMerge crash with illegal variable value

[Nat1405]

Relevant log:

out.fits -> /scratch/Titan/Merged_uncorrected/20140505_obs28_merged.fits
Killing IRAF task `imcombine'
Traceback (most recent call last):
  File "/opt/conda/envs/iraf27/bin/runNifty", line 38, in <module>
    nifty.nifsPipeline.start(sys.argv[2:])
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/nifty/pipeline/nifsPipeline.py", line 223, in start
    nifsMerge.run()
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/nifty/pipeline/steps/nifsMerge.py", line 111, in run
    finalMergeCubes(mergeType, over)
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/nifty/pipeline/steps/nifsMerge.py", line 489, in finalMergeCubes
    iraf.imcombine(inputstring, output = 'temp_merged'+gratlist[n][0]+'.fits', combine = mergeType, offsets = 'waveoffsets'+grat[0]+'.txt')
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/pyraf/iraftask.py", line 767, in __call__
    return self.run(*args, **kw)
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/pyraf/iraftask.py", line 359, in run
    self._run(redirKW, specialKW)
  File "/opt/conda/envs/iraf27/lib/python2.7/site-packages/pyraf/iraftask.py", line 811, in _run
    "\n"+str(value))
stsci.tools.irafglobals.IrafError: Error running IRAF task imcombine
IRAF task terminated abnormally
ERROR (603, "Parameter not a legal boolean (try 'yes' or 'no') (project)")