[Ros-release] MsgSrvDoc execution failed

I'm attempting to add automated message documentation using MsgSrvDoc to
the nmea_msgs package I released for Hydro. It is both released and in the
doc index (but it is new in Hydro, so is not present in Groovy or earlier).

When I add the <<MsgSrvDoc(nmea_msgs)>> call to
http://ros.org/wiki/nmea_msgs , the macro simply returns with the
following error:

<<MsgSrvDoc: execution failed [local variable 'url' referenced before
assignment] (see also the log)>>

I can't find any documentation on that macro, so I have no idea if I'm
missing some other step in order to use it on my own message package. I've
emulated what was done on both the sensors_msgs and tf2_msgs pages, so I
assumed it would work.

Is there something else I need to do to use the MsgSrvDoc macro?

- Eric