eclipseWtp does not work properly when a dependency resolves to a location for which no Eclipse variable has been defined

As talked to Adam in Mirc, using this gradle build file: http://gist.github.com/532185, and uncommenting the apply war, we have this stacktrace: http://pastie.org/1098680

This only happens because of our compile files(fileTree(dir: lib as File, includes: ['*.jar']))

The proposed workaround by adam that works is as follows:
eclipseWtp

{ variables['PROJECT_DIR'] = projectDir.absolutePath }