PYro4 v4.17 and HMAC_KEY error: hmac key config not symmetric

Hi,
I'm using Pyro4 v4.17 and I've some problems setting HMAC_KEY for
authentication.
To be more specific, whatever Pyro4.config.HMAC_KEY I set, running the
server code, I always receive the error:
"Pyro4.errors.comunicationError: cannot connect: hmac key config not
symmetric".
Note that my server Pyro4 code works normally if I comment the
HMAC_KEY setting and I can connect through a client. Nameserver
service is correctly running.
I tried to set Pyro4.config.HMAC_KEY both with a string or HMAC object
but same result.

An extract of my very simple code:

-------------------------------- BEGIN ------------------------------------
import Pyro4

def main():
    # create object of MyClass
    termos=MyClass();
    # configure for address binding
    Pyro4.config.HOST="x.x.x.x"
    #key and HMAC
    Pyro4.config.HMAC_KEY='the_same_string_for_server_and_client'
    # start Pyro4 daemon
    daemon=Pyro4.Daemon();
    # find the name server
    ns=Pyro4.locateNS();
    # remotize the object
    uri=daemon.register(termos);
    # register the object with a name in the name server
    ns.register("termos", uri);
    # start Pyro4 loop
    daemon.requestLoop();

if __name__=="__main__":
    main()

-------------------------- END -----------------------------------------------


Thanks in advance

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