Python decorator breaks when mutating parameters?

Question

I cannot understand what goes wrong in the following simple example in python 3.6.8:

import functools

def test(cache=False):
    def decorator_init(func):
        @functools.wraps(func)
        def wrapper_init(*args, **kwargs):
            print(cache)
            #cache = False
        return wrapper_init
    return decorator_init

@test(cache=False)
def main():
    return None

main()

Uncommenting cache = False will cause this program to fail saying that

Traceback (most recent call last):
  File "C:/.../debug.py", line 16, in <module>
    main()
  File "C:/.../debug.py", line 7, in wrapper_init
    print(cache)
UnboundLocalError: local variable 'cache' referenced before assignment

Why does attempting to mutate this variable lead to it being referenced before assignment, but otherwise it's fine to call it in a print statement?