Fatal Runtime Error when Iterating over Set Union

[JeffCarpenter]

% RUST_BACKTRACE=1 python -c 'from rust_fst import Set; it = Set.from_iter(["a", "b"]).union(Set.from_iter([])); list(it)'       
thread '<unnamed>' panicked at 'called `Result::unwrap()` on an `Err` value: NulError(0, [0])', ../src/libcore/result.rs:785
stack backtrace:
   1:     0x7fd6c57049d0 - std::sys::backtrace::tracing::imp::write::h9fb600083204ae7f
   2:     0x7fd6c57125eb - std::panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::hca543c34f11229ac
   3:     0x7fd6c5712223 - std::panicking::default_hook::hc2c969e7453d080c
   4:     0x7fd6c56d7b78 - std::panicking::rust_panic_with_hook::hfe203e3083c2b544
   5:     0x7fd6c5712831 - std::panicking::begin_panic::h4889569716505182
   6:     0x7fd6c56d9d8a - std::panicking::begin_panic_fmt::h484cd47786497f03
   7:     0x7fd6c57127ce - rust_begin_unwind
   8:     0x7fd6c572627f - core::panicking::panic_fmt::h257ceb0aa351d801
   9:     0x7fd6c5689ebe - core::result::unwrap_failed::h6b97717ae28838df
  10:     0x7fd6c5699c11 - fst_set_union_next
  11:     0x7fd6c59e7def - ffi_call_unix64
                        at ../src/x86/unix64.S:76
  12:     0x7fd6c59e7857 - ffi_call
                        at ../src/x86/ffi64.c:525
  13:     0x7fd6c5c05d63 - cdata_call
                        at c/_cffi_backend.c:3025
  14:           0x459892 - _PyObject_FastCallDict
  15:           0x54f116 - <unknown>
  16:           0x553aae - _PyEval_EvalFrameDefault
  17:           0x54e4c7 - <unknown>
  18:           0x5582c1 - _PyFunction_FastCallDict
  19:           0x459c10 - _PyObject_Call_Prepend
  20:           0x459892 - _PyObject_FastCallDict
  21:           0x4e2713 - <unknown>
  22:           0x4915bc - <unknown>
  23:           0x491a68 - <unknown>
  24:           0x4da8d6 - <unknown>
  25:           0x459892 - _PyObject_FastCallDict
  26:           0x54f116 - <unknown>
  27:           0x553aae - _PyEval_EvalFrameDefault
  28:           0x54efc0 - <unknown>
  29:           0x54ff72 - PyEval_EvalCode
  30:           0x42c379 - PyRun_SimpleStringFlags
  31:           0x441177 - Py_Main
  32:           0x421f63 - main
  33:     0x7fd6c61de1c0 - __libc_start_main
  34:           0x422019 - _start
  35:                0x0 - <unknown>
fatal runtime error: failed to initiate panic, error 5
zsh: abort (core dumped)  RUST_BACKTRACE=1 python -c