How to convert a Python 'bytes' object to ctypes 'POINTER(c_byte)'

I want to write a ctypes wrapper for a C function with this signature:

int utp_process_udp(utp_context *ctx, const byte *buf, size_t len, const struct sockaddr *to, socklen_t tolen);

And to have some type checking I’ve added this to my Python wrapper:

libutp.utp_process_udp.argtypes = [POINTER(UtpContext), POINTER(c_byte), c_size_t, POINTER(sockaddr_in), c_int]

But when I pass a bytes object as the buf argument I get this error:

Traceback (most recent call last):
  File "ucat.py", line 269, in <module>
    main()
  File "ucat.py", line 251, in main
    network_loop()
  File "ucat.py", line 138, in network_loop
    utp.utp_process_udp(ctx, data, addr)
  File "/home/mostafa/source/pyutp/utp.py", line 159, in utp_process_udp
    POINTER(c_byte)(data), c_size_t(len(data)),
TypeError: expected c_byte instead of bytes

If I don’t set argtypes I can successfully call the function though, so obviously the bytes object can be passed to this function. Is there anything I should do to let ctypes know what I’m doing? Some sort of cast perhaps? (I have tried using the ctypes.cast function but that did not work.)