Allowing in-channel registration from a specific IP

I'm trying to get ejabberd to allow in-channel registration from a single IP address using mod_register and ip_access. However, when I try to do an in-channel registration from that IP address using Adium on OS X, I get a 503 "service unavailable" status message, and the following is printed in /etc/ejabberd/ejabberd.log:

=INFO REPORT==== 2015-09-24 13:13:19 ===
I(<0.443.0>:ejabberd_listener:281) : (#Port<0.1968>) Accepted connection {{MY IP},59327} -> {{ANOTHER IP},5222}

=ERROR REPORT==== 2015-09-24 13:13:22 ===
E(<0.470.0>:ejabberd_hooks:335) : {function_clause,
                                   [{mod_register,'-get_ip_access/1-fun-0-',
                                     "7",
                                     [{file,"mod_register.erl"},{line,564}]},
                                    {lists,flatmap,2,
                                     [{file,"lists.erl"},{line,1235}]},
                                    {mod_register,try_register,5,
                                     [{file,"mod_register.erl"},{line,328}]},
                                    {mod_register,
                                     try_register_or_set_password,9,
                                     [{file,"mod_register.erl"},{line,281}]},
                                    {mod_register,
                                     unauthenticated_iq_register,4,
                                     [{file,"mod_register.erl"},{line,78}]},
                                    {ejabberd_hooks,run_fold1,4,
                                     [{file,"ejabberd_hooks.erl"},{line,331}]},
                                    {ejabberd_c2s,
                                     process_unauthenticated_stanza,2,
                                     [{file,"ejabberd_c2s.erl"},{line,2179}]},
                                    {ejabberd_c2s,wait_for_feature_request,2,
                                     [{file,"ejabberd_c2s.erl"},{line,731}]}]}
running hook: {c2s_unauthenticated_iq,
                  ["SERVER IP",
                   {iq,"purple5b6ad190",set,"jabber:iq:register",[],
                       {xmlelement,"query",
                           [{"xmlns","jabber:iq:register"}],
                           [{xmlelement,"username",[],
                                [{xmlcdata,<<"test-new">>}]},
                            {xmlelement,"password",[],
                                [{xmlcdata,<<"test">>}]}]}},
                   {{MY IP},59327}]}

I've never worked with ejabberd or Erlang before, so this is all Greek to me. What's happening? Why can't I perform in-channel registration from that IP? What can I do to allow it?

Thanks