maven assembly plugin tgz - no such archiver - maven

Question

I need to do a really simple tar action using the maven-assembly-plugin.
My file extension must be .tgz not tar.gz
Here is my pom.xml
<project>
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<version>1</version>
<build>
<plugins>
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<version>2.2.1</version>
<executions>
<execution>
<id>default-cli</id>
<phase>package</phase>
<goals>
<goal>single</goal>
</goals>
<configuration>
<finalName>${project.artifactId}</finalName>
<descriptors>
<descriptor>dist-all.xml</descriptor>
</descriptors>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
and here is my assembly description file
<?xml version="1.0" encoding="UTF-8"?>
<assembly>
<id>default-cli</id>
<formats>
<format>tgz</format>
</formats>
<includeBaseDirectory>false</includeBaseDirectory>
<fileSets>
<fileSet>
<directory>src/app</directory>
<outputDirectory>target</outputDirectory>
</fileSet>
</fileSets>
</assembly>
when I run mvn clean install it generates the following error:
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-assembly-plugin:2.2.1:single (default-cli) on project my-app: Failed to create assembly: Unable to obtain archiver for extension 'tgz', for assembly: 'default-cli': No such archiver: 'tgz'. -> [Help 1]
[ERROR]
According to the maven page I am allowed 'tgz' format.
https://maven.apache.org/plugins/maven-assembly-plugin/assembly.html
What have I missed?
I'd be grateful for any help,