Strange IndexError in TransformMatchingTex | Manim

Question

I'm trying to run TransformMatchingTex between two values:

start = Tex(r"a", "b", r"c")\   # initial part of transform
end = Tex(r"dad", "b", r"fcf")\ # result of transform
self.play(FadeIn(start))        # start rendering of start

Here's the way I'm doing the TransformMatchingTex function:

self.play(TransformMatchingTex(
            start, end,
            key_map={
                r"a": r"dad",
                r"c": r"fcf",
            }
        ))

When I run this, I get this error:

  File "filename", line 81, in construct
    self.play(TransformMatchingTex(
              ^^^^^^^^^^^^^^^^^^^^^
  File "...manimlib\animation\transform_matching_parts.py", line 135, in __init__
    *self.matching_blocks(source, target, matched_keys, key_map),
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...manimlib\animation\transform_matching_parts.py", line 166, in matching_blocks
    if target[j] in sub_target.family_members_with_points():
       ~~~~~~^^^
  File "...manimlib\mobject\svg\string_mobject.py", line 552, in __getitem__
    return super().__getitem__(value)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "...manimlib\mobject\mobject.py", line 354, in __getitem__
    return self.split().__getitem__(value)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
IndexError: list index out of range

The code above is what I have after a couple fixes, which had the exact same error before.