Invalid file type (OpnApplFil/OPNPTI) when saving

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I tried to save a file recently with the following script:

import sys, os
PSSE = "c:/Program Files/pti/psse32/pssbin"
sys.path.append(PSSE)
os.environ['PATH'] += PSSE + ';'

import psspy
psspy.throwPsseExceptions = True

psspy.newcase_2(titl1="an empty saved case")
psspy.bus_data_2(101)
psspy.save("empty.sav")

and PSSE shows this error:

Invalid file type. empty.sav (OpnApplFil/OPNPTI)
Traceback (most recent call last):
  File "top_secret_consulting_work.py", line 72 in <module>
    psspy.save("empty.sav")
  File ".\psspy.py", line 13500, in save
psspy.SaveError: save Error: ierr=3

When looking up in the API ierr=3 is:

error opening SFILE.

Anyone have any ideas about what the problem is?