dependencySets in a descriptor doesn't work anymore

I attached my descriptor file.
<dependencySets>
<dependencySet>
<outputDirectory>lib</outputDirectory>
<unpack>false</unpack>
<scope>runtime</scope>
<!--
how to exclude dependencies
<excludes>
<exclude>junit:junit</exclude>
</excludes>
-->
</dependencySet>
</dependencySets>

unzip -l on the assembly file show empty lib directory.
Olivier