mbunit test integration problem

I'm getting the following error when running an msbuild script that is supposed to run our unit tests written for mbunit:

Belos.ERP.Tests.dll File 'Belos.ERP.Tests.dll' is not supported by any installed test framework.  It will be ignored.

The simple build script looks like this:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <!-- This is needed by MSBuild to locate the Gallio task -->
  <UsingTask AssemblyFile="C:\Programme\Gallio\bin\Gallio.MSBuildTasks.dll" TaskName="Gallio" />
  <!-- Specify the test files and assemblies -->
  <ItemGroup>
    <TestFile Include="Belos.ERP.Webshop.Tests\bin\Release\Belos.ERP.Tests.dll" />
  </ItemGroup>
  <Target Name="RunTests">
    <Gallio IgnoreFailures="true" Files="@(TestFile)">
      <!-- This tells MSBuild to store the output value of the task's ExitCode property
                 into the project's ExitCode property -->
      <Output TaskParameter="ExitCode" PropertyName="ExitCode"/>
    </Gallio>
    <Error Text="Tests execution failed" Condition="'$(ExitCode)' != 0" />
  </Target>
</Project>

The test is found and Team City tries to run it, so I think that's not the issue. I'm not sure if this is a Team City or Gallio issue either. Searched up and down the internet and couldn't find a solution yet. Any ideas?