Nant and test fail

When I run tmockrunner from a Nant script as below, even if the tests fail, the build will not stop. Is there any way to achieve the stop of the build.

Thanks,

<exec program="tmockrunner">
<arg line="nunit-console.exe /xml:resultsRouterUnitTests.dll-results.xml ${debug-dir}${testproject}.dll"/>
</exec>