Can't run project that includes both Junit and TestNG testcases in surefire using 2 executions.

My project needs to use both Junit and TestNG.

I have surefire set up to successfully run Junit and TestNG tests with 2 executions.
This setup is described here: http://confluence.highsource.org/display/~lexi/How+to+run+both+JUnit+and+TestNG+with+maven-surefire-plugin

Cobertura works with either of the 2 executions on their own, but not with both enabled at the same time.
ie. If I comment one out, it works.

I have a testSuiteXml configuration for the testNG execution listing my testng suite files. The error referes to the testSuiteXml being a problem, but I think this is misleading. As I say, I can run testng or junit if the other is commented out. I can run both when cobertura is not included.

I get the following error:
--------------------------------------------------------------------------------------

Cobertura: Saved information on 50 classes.
Instrument time: 407ms

[INFO] Instrumentation was successful.
[INFO]
[INFO] — maven-resources-plugin:2.4.3:testResources (default-testResources) @ core —
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] Copying 5 resources
[INFO]
[INFO] — maven-compiler-plugin:2.3.2:testCompile (default-testCompile) @ core —
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] — maven-surefire-plugin:2.7.2:test (default-test) @ core —
[INFO] Surefire report directory: /home/alan/dev/workspaces/plumbunit/core/target/surefire-reports
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 12.681s
[INFO] Finished at: Sun Feb 13 23:42:33 GMT 2011
[INFO] Final Memory: 15M/79M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:2.7.2:test (default-test) on project core: suiteXmlFiles is configured, but there is no TestNG dependency -> [Help 1]

-----------------------------------------------------------
My Maven plugin config is below:
...
<surefire.version>2.7.2</surefire.version>
<cobertura.version>2.4</cobertura.version>
...

<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>cobertura-maven-plugin</artifactId>
<version>$

</version>
<configuration>
<instrumentation>

</instrumentation>
<check>
<branchRate>85</branchRate>
<lineRate>85</lineRate>
<haltOnFailure>true</haltOnFailure>
<totalBranchRate>85</totalBranchRate>
<totalLineRate>85</totalLineRate>
<packageLineRate>85</packageLineRate>
<packageBranchRate>85</packageBranchRate>
<regexes>
<regex>
<pattern>org.*</pattern>
<branchRate>90</branchRate>
<lineRate>80</lineRate>
</regex>
</regexes>
</check>
</configuration>
<executions>
<execution>
<goals>
<goal>clean</goal>
<goal>check</goal>
</goals>
</execution>
</executions>
</plugin>

<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-surefire-plugin</artifactId>
<version>$

</version>

<executions>
<execution>
<id>test-junit</id>
<phase>test</phase>
<goals>
<goal>test</goal>
</goals>
<configuration>
<testNGArtifactName>none:none</testNGArtifactName>
</configuration>
</execution>
<execution>
<id>test-testng</id>
<phase>test</phase>
<goals>
<goal>test</goal>
</goals>
<configuration>
<systemProperties>
<property>
<name>net.sourceforge.cobertura.datafile</name>
<value>target/cobertura/cobertura.ser</value>
</property>
</systemProperties>

<junitArtifactName>none:none</junitArtifactName>
<suiteXmlFiles>
<suiteXmlFile>$

/src/test/resources/META-INF/testng/core.xml</suiteXmlFile>
</suiteXmlFiles>
</configuration>
</execution>
</executions>
</plugin>
</plugins