Why is registering dissector w/(WTAP_ENCAP_USER0) in code not working?

edit

Register-handoff routine:

void proto_reg_handoff_con15(void) {

            dissector_handle_t con15_handle;

            con15_handle = create_dissector_handle(dissect_con15, proto_con15);
            dissector_add_uint("wtap_encap",WTAP_ENCAP_USER0,con15_handle);
        }

Register routine:

void proto_register_con15(void)
    {

        module_t *con15_module;
        proto_con15 = proto_register_protocol("con15 Protocol",
                                                "con15",
                                                "con15");

        proto_register_field_array(proto_con15, hf, array_length(hf));
        proto_register_subtree_array(ett, array_length(ett));

        con15_module = prefs_register_protocol(proto_con15, NULL);

        con15_address_type = address_type_dissector_register("AT_CON15", "CON 15 Address", con15_addr_to_str, con15_addr_str_len, NULL, NULL, NULL, NULL, NULL);

        con15_dissector_table = register_dissector_table("con15.addr",
            "CON 15", proto_con15, FT_UINT8, BASE_DEC);
    }