When using bluez-pin via dbus (manually started bluez-pin --dbus for now; see
related bug 185208) and attempting to pair with a device, I get the following in
the syslog:

Mar 11 15:39:55 caladan hcid[24662]: pin_code_request (sba=00:14:A4:D4:CE:99,
dba=00:0F:86:26:BF:9B)
Mar 11 15:39:55 caladan hcid[24662]: org.bluez.Error.WrongArgs: Byte array
expected, other type given

and the following outputted by bluez-pin:

24678: arguments to dbus_message_iter_get_fixed_array() were incorrect,
assertion "(subtype == DBUS_TYPE_INVALID) || dbus_type_is_fixed (subtype)"
failed in file dbus-message.c line 1743.
This is normally a bug in some application using the D-BUS library.

Consequently I cannot properly pair a device using the default configuration for
hcid.  If I edit hcid.conf and change:

    # PIN helper
    #pin_helper /usr/bin/bluepin;

    # D-Bus PIN helper
    dbus_pin_helper;

to this:

    # PIN helper
    pin_helper /usr/bin/bluepin;

    # D-Bus PIN helper
    #dbus_pin_helper;

(i.e. bypass dbus) then I am properly prompted for a passkey.