Maven Jetty plugin stopPort and stopKey missing or invalid

I am learning Maven and have encountered a problem. When I try to do mvn clean install with my webapp i get the error saying that parameters stopPort and stopKey are missing or invalid. Here is what pom.xml looks like:

    <plugin>
       <groupId>org.mortbay.jetty</groupId>
       <artifactId>maven-jetty-plugin</artifactId>
       <version>6.1.17</version>
       <executions>
         <execution>
            <id>start-jetty</id>
            <phase>pre-integration-test</phase>
            <goals>
              <goal>run</goal>
            </goals>
            <configuration>
              <scanIntervalSeconds>0</scanIntervalSeconds>
              <stopPort>9999</stopPort>
              <stopKey>foo</stopKey>
              <daemon>true</daemon>
            </configuration>
         </execution>
         <execution>
            <id>stop-jetty</id>
            <phase>post-integration-test</phase>
            <goals>
              <goal>stop</goal>
            </goals>
         </execution>
       </executions>
    </plugin>

Any idea what could cause that? Thx in advance.