Maven-pdf plugin fails with href link inside an xdoc file

The FOP convertion crashes when using href link in an xdoc file.

--the xdoc file:
...
<subsection name="The Configuration schema">
<a href="schema/conf.html">The schema description</a>
</subsection>
...

--the fo result:
...
<fo:block font-family="sans-serif" color="#000036" space-before.optimum="1.5em" space-before.minimum="1.2em" space-before.maximum="2.0em" font-size="12pt">The Configuration schema</fo:block>
<fo:basic-link external-destination="schema/conf.html"><fo:inline color="blue">The schema description</fo:inline></fo:basic-link>
...

--the error:
...
[java] [ERROR] file:/C:/projets/LSM/target/docs/project.fo:50:61 inline form
atting objects cannot be directly under flow
[java] org.apache.fop.apps.FOPException: file:/C:/projets/LSM/target/docs/pr
oject.fo:50:61 inline formatting objects cannot be directly under flow
[java] at org.apache.fop.fo.flow.Inline.<init>(Inline.java:77)
[java] at org.apache.fop.fo.flow.BasicLink.<init>(BasicLink.java:75)
[java] at org.apache.fop.fo.flow.BasicLink$Maker.make(BasicLink.java:65
)
[java] at org.apache.fop.fo.FOTreeBuilder.startElement(FOTreeBuilder.ja
va:352)
[java] at org.apache.xerces.parsers.AbstractSAXParser.startElement(Unkn
own Source)
[java] at org.apache.xerces.impl.XMLNSDocumentScannerImpl.scanStartElem
ent(Unknown Source)
[java] at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$Fragmen
tContentDispatcher.dispatch(Unknown Source)
[java] at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDoc
ument(Unknown Source)
[java] at org.apache.xerces.parsers.XML11Configuration.parse(Unknown So
urce)
[java] at org.apache.xerces.parsers.DTDConfiguration.parse(Unknown Sour
ce)
[java] at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
[java] at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Sou
rce)
[java] at org.apache.fop.apps.Driver.render(Driver.java:498)
[java] at org.apache.fop.apps.CommandLineStarter.run(CommandLineStarter
.java:106)
[java] at org.apache.fop.apps.Fop.main(Fop.java:62)
...

=> It works by modifying the xslt.
I add a fo:block element surrounding the fo:basic-link element (patch attached)

...
<xsl:template match="a[@href]">
<fo:block>
<fo:basic-link external-destination="

">
<fo:inline color="blue">
<xsl:apply-templates />
</fo:inline>
</fo:basic-link>
</fo:block>
</xsl:template>
...