pysim_typeerror

(env) ➜  counter ./tb_counter.py 4
Traceback (most recent call last):
  File "/Users/sam/swdev/scratchpad/gateware/amaranth_learning/counter/./tb_counter.py", line 47, in <module>
    with sim.write_vcd("counter.vcd"):
  File "/opt/homebrew/Cellar/[email protected]/3.11.3/Frameworks/Python.framework/Versions/3.11/lib/python3.11/contextlib.py", line 137, in __enter__
    return next(self.gen)
           ^^^^^^^^^^^^^^
  File "/Users/sam/swdev/scratchpad/gateware/amaranth_learning/env/lib/python3.11/site-packages/amaranth/sim/pysim.py", line 329, in write_vcd
    vcd_writer = _VCDWriter(self._fragment,
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/sam/swdev/scratchpad/gateware/amaranth_learning/env/lib/python3.11/site-packages/amaranth/sim/pysim.py", line 94, in __init__
    if re.search(r"[ \t\r\n]", var_name):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/[email protected]/3.11.3/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/__init__.py", line 176, in search
    return _compile(pattern, flags).search(string)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: expected string or bytes-like object, got 'NoneType'