mvn help:evaluate does not return an entry in a LinkedHashMap, even though the item is present

[INFO] Enter the Maven expression i.e. $

{project.groupId} or 0 to exit?:
${project.build.pluginsAsMap}
<linked-hash-map>

[Content erased for brevity]

<entry>
<string>org.apache.maven.plugins:maven-help-plugin</string>
<plugin>
<inheritanceApplied>true</inheritanceApplied>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-help-plugin</artifactId>
<version>2.1</version>
<extensions>false</extensions>
<executions/>
<dependencies/>
<key>org.apache.maven.plugins:maven-help-plugin</key>
</plugin>
</entry>
</linked-hash-map>

But:

[INFO] Enter the Maven expression i.e. ${project.groupId}

or 0 to exit?:
$

{project.build.pluginsAsMap(org.apache.maven.plugins:maven-help-plugin)}

[INFO]
null object or invalid expression

Am I doing something wrong? I'm using the key, as described here: http://jira.codehaus.org/browse/PLXUTILS-37