More experiments

from collections import namedtuple
foo = namedtuple('foo', '')
a = [] + foo()
print (a, type(a), len(a))
# () <class 'tuple'> 0

ie, a standard empty tuple, whereas
a = [1,1] + foo()
crashes immediately. So the behavior of list()+namedtuple depends on the length of the list.

There are also some funny interactions. Adding
try:
    a = foo()+[]
except TypeError:
    print("correct TypeError")

after the 'foo = ' line in my original 5 line example causes the final print to crash, whereas adding the same 4 lines to the 4 line example at the beginning of this message does not obviously change anything.

David, since you omitted all details, I wonder if you tested in batch mode, as I did, and hence never tried to print the malformed object, or used different OS or hardware.