error when running taup.tt

[Zaur2302]

The code stops at:
.....
20 0.01
top_p 1693.0749299719887 bot_p 1604.3571428571424 radius_of_planet 6371.0 top_depth 19.5914894768942 bot_depth 22.528838091875922
b 116.35465802326787
Traceback (most recent call last):

Traceback (most recent call last):
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/tau_model.py", line 233, in load_from_depth_cache
value = self._depth_cache.pop(depth)
KeyError: 20

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "taup_tt.py", line 30, in
arrivals = model.get_travel_times(source_depth_in_km=dep, distance_in_degree=dist, phase_list=["P","p","S","s"])
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/tau.py", line 700, in get_travel_times
tt.run()
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/taup_time.py", line 37, in run
self.depth_correct(self.source_depth, self.receiver_depth)
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/taup_time.py", line 49, in depth_correct
self.depth_corrected_model = self.model.depth_correct(depth)
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/tau_model.py", line 226, in depth_correct
return self.load_from_depth_cache(depth)
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/tau_model.py", line 235, in load_from_depth_cache
value = self._load_from_depth_cache(depth)
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/tau_model.py", line 246, in _load_from_depth_cache
depth_corrected = self.split_branch(depth)
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/tau_model.py", line 272, in split_branch
split_info = out_s_mod.split_layer(depth, is_p_wave)
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/slowness_model.py", line 1762, in split_layer
p = evaluate_at_bullen(s_layer, depth, self.radius_of_planet)
File "/home/user/.local/lib/python3.8/site-packages/obspy/taup/slowness_layer.py", line 289, in evaluate_at_bullen
or math.isinf(a_denominator)
UnboundLocalError: local variable 'a_denominator' referenced before assignment