Assembly task does not include deeper nested modules

If one of the modules of the root pom is itself a "pom" packaging project, specifying

<moduleSets>
<moduleSet>
<binaries>
<outputDirectory>lib</outputDirectory>
<includeDependencies>true</includeDependencies>
<unpack>false</unpack>
</binaries>
</moduleSet>
</moduleSets>

in the descriptor fails with

[INFO] Included module: dcm4che:dcm4che-tool:pom:1 does not have an artifact with a file. Please ensure the package phase is run before the assembly is generated.

Excluding it by (e.g.)

<moduleSets>
<moduleSet>
<excludes>
<exclude>dcm4che:dcm4che-tool</exclude>
</excludes>
:

excludes also the artifacts from its sub-modules.
Also listing such deeper nested modules by its groupId:artifactId as <includes> (e.g

<moduleSets>
<moduleSet>
<includes>
<include>dcm4che:dcm4che-tool-dcm2txt</include>
<include>dcm4che:dcm4che-tool-dcm2xml</include>
<include>dcm4che:dcm4che-tool-pdf2dcm</include>
<include>dcm4che:dcm4che-tool-xml2dcm</include>
:

does not help.