TypeError: can only concatenate str (not "list") to str

[kenorb]

$ exrex -l 4 '[bcdfghjklmnpqrstvwxyz]*(?:[aeiouy]{1}(?:[bcdfghjklmnpqrstvwxyz]*(?:e(?=$))?)|[aeiou]*(?:ai|ay|ea|ee|ie|oa|oe|ue|ui|oo|oi|oy|ou|ow|au|aw|ar|er|ir|or|ur|air|ear|eer))[bcdfghjklmnpqrstvwxyz]*'
...
azzx
azzy
azzzTraceback (most recent call last):
  File "/home/kenorb/.local/bin/exrex", line 8, in <module>
    sys.exit(__main__())
  File "/home/kenorb/.local/bin/exrex.py", line 539, in __main__
    for s in g:
  File "/home/kenorb/.local/bin/exrex.py", line 134, in prods
    for o in orig:
  File "/home/kenorb/.local/bin/exrex.py", line 165, in concit
    for b in _gen(s, limit, grouprefs=grouprefs):
  File "/home/kenorb/.local/bin/exrex.py", line 143, in prods
    for i in ret:
  File "/home/kenorb/.local/bin/exrex.py", line 157, in ggen
    yield a + b
TypeError: can only concatenate str (not "list") to str

Although this works fine:

exrex -l 4 '([bcdfghjklmnpqrstvwxyz]*([aeiouy]([bcdfghjklmnpqrstvwxyz]*e?)|[aeiou]*(ai|ay|ea|ee|ie|oa|oe|ue|ui|oo|oi|oy|ou|ow|au|aw|ar|er|ir|or|ur|air|ear|eer))[bcdfghjklmnpqrstvwxyz]*)'