Error when running a GWTTestCase using maven gwt plugin

I've created a test which extends GWTTestCase but I'm getting this error:

mvn integration-test gwt:test
...
Running com.myproject.test.ui.GwtTestMyFirstTestCase
Translatable source found in...                       
[WARN] No source path entries; expect subsequent failures
[ERROR] Unable to find type 'java.lang.Object'
[ERROR] Hint: Check that your module inherits 'com.google.gwt.core.Core' either directly or indirectly (most often by inheriting module 'com.google.gwt.user.User')
Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 2.1 sec <<< FAILURE!

GwtTestMyFirstTestCase.java is in /src/test/java, while the GWT module is located in src/main/java. I assume this shouldn't be a problem.

I've done everything required according to http://mojo.codehaus.org/gwt-maven-plugin/user-guide/testing.html and of course that my gwt module already has com.google.gwt.core.Core indirectly imported.

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.myproject</groupId>
<artifactId>main</artifactId>
<packaging>jar</packaging>
<version>0.0.1-SNAPSHOT</version>
<name>Main Module</name>

<properties>
    <gwt.module>com.myproject.MainModule</gwt.module>
</properties>

<parent>
    <groupId>com.myproject</groupId>
    <artifactId>app</artifactId>
    <version>0.1.0-SNAPSHOT</version>
</parent>

<dependencies>

    <dependency>
        <groupId>com.myproject</groupId>
        <artifactId>app-commons</artifactId>
        <version>