can't inherit gwt maps module

I've been trying to build a simple GWT app using GWT-maps.jar, like
the one at: http://gwttutorials.com/2009/07/28/setting-up-google-maps-with-gwt/

Despite following the instructions, I still get error messages like
this one:
No source code is available for type
com.google.gwt.maps.client.geom.LatLng; did you forget to inherit a
required module?

I cannot find anything wrong with my application.gwt.xml file, which
includes the following lines:

<inherits name='com.google.gwt.maps.GoogleMaps'/>
<script src='http://maps.google.com/maps?gwt=1&amp;file=api&amp;v=2'/
>

I have also made sure that the gwt-maps.jar is on my build path.

I would appreciate any guidance about where to look for problems.
Thanks.

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