"validity error" on fresh build

[bagratte]

16:38 ~/src % git clone [email protected]:sonatype/maven-example-en.git
Cloning into 'maven-example-en'...
remote: Enumerating objects: 8587, done.
remote: Total 8587 (delta 0), reused 0 (delta 0), pack-reused 8587
Receiving objects: 100% (8587/8587), 88.01 MiB | 694.00 KiB/s, done.
Resolving deltas: 100% (4234/4234), done.
16:40 ~/src % cd maven-example-en 
16:42 ~/src/maven-example-en % ./build.sh 
Building PDF
I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
target/book-mvnex.xml:2: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
D DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
                                                                               ^
target/book-mvnex.xml:6: validity error : Validation failed: no DTD found !
<book lang="en">
               ^
16:42 ~/src/maven-example-en %