Why am I getting a bad file descriptor error while calling a C function to write to log?

This function calls a C function to write to log from Python. It does print the log text but throws an error after it. What am I doing wrong?

def py_log_callback(msg, level):
    if not string_at(msg):
        return
    msg = "Log entry: {}".format(string_at(msg).decode())
    print(msg)

log_callback = CFUNCTYPE(None, c_char_p, c_int)(py_log_callback)
lib = CDLL(libpath)
lib.Foo(log_callback, DEBUG_LEVEL)

Output:

Log entry: <log text here>

Traceback (most recent call last):
  File "_ctypes/callbacks.c", line 234, in 'calling callback function'
  File "test.py", line 36, in py_log_callback
    print(msg)
OSError: [Errno 9] Bad file descriptor