[maven] bad path element "": no such file or directory

Our config options are (as in the effective POM):

 

<plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>2.5.1</version>
            <inherited>true</inherited>
            <configuration>
                  <fork>true</fork>
                  <showDeprecation>true</showDeprecation>
                  <showWarnings>true</showWarnings>
                  <source>1.6</source>
                  <target>1.6</target>
                  <compilerVersion>1.6</compilerVersion>
                  <compilerArgument>-Xlint:all</compilerArgument>
                  <testCompilerArgument>-Xlint:all</testCompilerArgument>
            </configuration>
</plugin>