0% code coverage reported with maven-surefire-plugin 2.5

I'm using maven-surefire-plugin (2.5) and cobertura-maven-plugin (2.3). When I run "mvn clean sonar:sonar" at the command line I can see that unit tests are run successfully, but in the Sonar dashboard the coverage is reported as 0%. If I remove the maven-surefire-plugin from my pom and try again, the Sonar dashboard displays the expected non-zero amount of code coverage. This is happening with Sonar 2.0.1, but it also was a problem with Sonar 1.11.1.