>>> eval("f'\\\n'")
python: Python/ast.c:4906: FstringParser_ConcatFstring: Assertion `!state->last_str || PyUnicode_GET_LENGTH(state->last_str) != 0' failed.
[2]    12810 abort (core dumped)  ./python

The problem is that some literal strings are zero length. The assert can be deleted.

New changeset 11e97f2f80bf65cc828c127eafc95229df35d403 by Serhiy Storchaka (ericvsmith) in branch 'master':
bpo-30682: Removed a too-strict assertion that failed for certain f-strings. (#2232)
https://github.com/python/cpython/commit/11e97f2f80bf65cc828c127eafc95229df35d403

New changeset 2eca5b465f7404cc8484457b7966f828f434ec20 by ericvsmith (Serhiy Storchaka) in branch '3.6':
[3.6] bpo-30682: Removed a too-strict assertion that failed for certain f-strings. (GH-2232) (#2242)
https://github.com/python/cpython/commit/2eca5b465f7404cc8484457b7966f828f434ec20

Before the patch, eval("f'\\\n'") gives me the same assertion failure, with an IDLE freeze instead of core dump equivalent, but is seems that the string should be of length 2.

>>> len('\\\n')
2
>>> len(f'\\\n')
2

Terry:

The eval is important. The bug was in evaluating an f-string that consisted of two bytes: a backslash followed by a newline.

And just as:
eval("'\\\n'") == ''  # len == 0
so should
eval("f'\\\n'") == ''  # len == 0

It's the second one that was throwing the assertion. The parser was seeing these bytes:
f  0x66
'  0x27
\  0x5c
nl 0xa
'  0x27
and behaving badly (to say the least) by asserting.

Without the eval, I can't think of a way to have a string consisting of those two bytes, but I assume someone who's trickier than I can come up with a way.