pkt_xmt_gr38 SWIG error

[gnuradio]

First, many thanks to the folks who contributed to the new tutorials; 
fantastic to see this material out there.

While working with pkt_xmt_gr38, I get this cryptic "Runtime Error." I 
did find a discussion about a different issue with this script from last 
November, but this one is different:

handler caught exception:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/gnuradio/gr/gateway.py", line 78, 
in eval
    try: self._callback(arg)
  File "/home/user/Documents/packet_format_gr38.py", line 23, in 
handle_msg
    inMsg = pmt.to_python (msg)
  File "/usr/lib/python3/dist-packages/pmt/pmt_to_python.py", line 126, 
in pmt_to_python
    return to_python(p)
  File "/usr/lib/python3/dist-packages/pmt/pmt_to_python.py", line 57, 
in pmt_to_dict
    for i in range(pmt.length(items)):
  File "/usr/lib/python3/dist-packages/pmt/pmt_swig.py", line 2647, in 
length
    return _pmt_swig.length(v)
RuntimeError
thread[thread-per-block[12]: <block Packet Format GR38(3)>]: SWIG 
director method error. Error detected when calling 'feval_p.eval'


Really don't know where to go with just "RuntimeError." I've verified 
that msg is not empty, but that's about all I can figure to do. Any 
ideas on what to try next?

Environment is Debian Bullseye gr 3.8.

Best,
Mike