Problems dissecting FT_ETHER

Hello, I need to dissect a MAC address, but the format FT_ETHER gives me a hard time. I looked into packet_eth.c and used this code:

static int hf_mac = -1;
static void dissect_xxx(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree) {
    if (tree) {
        const guint8 *address;
        address = tvb_get_ptr(tvb, 0, 6);
    proto_tree_add_ether(tree, hf_mac, tvb, 0, 6, address);
}

}
void
proto_register_xxx(void) {
static hf_register_info hf[] = {
{&hf_mac,
{"MAC", "xxx.mac", FT_ETHER, BASE_NONE, NULL, 0x00, NULL, HFILL}},
};
[...]

}

When dissecting the packet, wireshark gives me this error:

Expert Info (Error/Malformed): proto.c:2672: failed assertion “hfinfo->type == FT_ETHER”

Why is that? I declared it as FT_ETHER…

Wireshark 1.6.4, dissector as a plugin.