Crash with map in function expression and sum with non-numerical arguments

The following program works:

mask = (b: true, false, true)
len = sum(map((x: if(x, 1, 0)), mask))
print(len)

but if I define a function:

mask = (b: true, false, true)
lenf(z) = sum(map((x: if(x, 1, 0)), z))
print(lenf(mask))

I get this error:

Traceback (most recent call last):
  File "vre-language/src/virtmat/language/utilities/errors.py", line 87, in wrapper
    return func(*args, **kwargs)
  File "../scripts/run_model.py", line 56, in run_instant_deferred
    return meta.model_from_file(clargs.model_file, deferred_mode=deferred,
  File "jupyter-tensorflow-2023-01-02/lib64/python3.8/site-packages/textx/metamodel.py", line 658, in model_from_file
    return self.internal_model_from_file(
  File "jupyter-tensorflow-2023-01-02/lib64/python3.8/site-packages/textx/metamodel.py", line 712, in internal_model_from_file
    p(model, self)
  File "vre-language/src/virtmat/language/metamodel/function.py", line 38, in function_call_processor
    call.__expr = deepcopy(call.function.expr)
  File "/usr/lib64/python3.8/copy.py", line 172, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/lib64/python3.8/copy.py", line 270, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/lib64/python3.8/copy.py", line 146, in deepcopy
    y = copier(x, memo)
  File "/usr/lib64/python3.8/copy.py", line 230, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib64/python3.8/copy.py", line 172, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/lib64/python3.8/copy.py", line 270, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/lib64/python3.8/copy.py", line 146, in deepcopy
    y = copier(x, memo)
  File "/usr/lib64/python3.8/copy.py", line 230, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib64/python3.8/copy.py", line 172, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/lib64/python3.8/copy.py", line 270, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/lib64/python3.8/copy.py", line 146, in deepcopy
    y = copier(x, memo)
  File "/usr/lib64/python3.8/copy.py", line 230, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib64/python3.8/copy.py", line 172, in deepcopy
    y = _reconstruct(x, memo, *rv)
  File "/usr/lib64/python3.8/copy.py", line 270, in _reconstruct
    state = deepcopy(state, memo)
  File "/usr/lib64/python3.8/copy.py", line 146, in deepcopy
    y = copier(x, memo)
  File "/usr/lib64/python3.8/copy.py", line 230, in _deepcopy_dict
    y[deepcopy(key, memo)] = deepcopy(value, memo)
  File "/usr/lib64/python3.8/copy.py", line 161, in deepcopy
    rv = reductor(4)
TypeError: cannot pickle '_io.TextIOWrapper' object

It is striking that sum(mask) works - it returns the number of true elements which is the behavior in Python but should not be in the DSL. I thought that only numerical types are allowed in sum. For non-arithmetic types either reduce() should be used or a map converting the booleans to 0 or 1 integers.