JMeter Plugin: NonGuiDriver NullPointerException & SOAPException

When using the Maven JMeter Plugin to try and send web service requests, I ran into the following exception:

  Error in NonGUIDriver java.lang.NullPointerException

My jmeter.log presented the following:

2011/04/27 14:20:46 ERROR - jmeter.save.SaveService: Conversion error com.thoughtworks.xstream.converters.ConversionException: org/apache/soap/SOAPException : org/apache/sp/SOAPException
---- Debugging information ----
message             : org/apache/soap/SOAPException
cause-exception     : java.lang.NoClassDefFoundError
cause-message       : org/apache/soap/SOAPException
first-jmeter-class  : org.apache.jmeter.save.converters.TestElementConverter.unmarshal(TestElementConverter.java:108)
class               : org.apache.jmeter.save.ScriptWrapper
required-type       : org.apache.jmeter.protocol.http.sampler.WebServiceSampler
path                : /jmeterTestPlan/hashTree/hashTree/hashTree[2]/WebServiceSampler
line number         : 741
-------------------------------
2011/04/27 14:20:46 ERROR - jmeter.JMeter:  java.lang.NullPointerException
        at org.apache.jmeter.gui.tree.JMeterTreeModel.addSubTree(JMeterTreeModel.java:91)
        at org.apache.jmeter.JMeter.run(JMeter.java:728)
        at org.apache.jmeter.JMeter.startNonGui(JMeter.java:706)
        ...

The solution was to add a dependency on soap:soap:2.3 to my plugin definition:

      <plugin>
        <groupId>org.apache.jmeter</groupId>
        <artifactId>maven-jmeter-plugin</artifactId>
        <version>1.0</version>
        <configuration>
          <reportDir>${reportDir}</reportDir>
        </configuration>
        <dependencies>          
          <dependency>
            <groupId>soap</groupId>
            <artifactId>soap</artifactId>
            <version>2.3</version>
          </dependency>
        </dependencies>
      </plugin>

After adding the dependency, the tests ran fine:

  Starting the test @ Wed Apr 27 14:42:57 EDT 2011 (1303929777586)
  Tidying up ...    @ Wed Apr 27 14:42:59 EDT 2011 (1303929779839)
  ... end of run