Qt Enginio (dev branch) failing to build "make html_docs"

Running "make html_docs" in qtenginio/ results in:

make -f Makefile prepare_docs && make -f Makefile generate_docs
make[1]: Entering directory `/home/jerome/build/dev/qtenginio'
cd src/ && ( test -e Makefile || /home/jerome/build/dev/qtbase/bin/qmake /home/jerome/gerrit/dev/qtenginio/src/src.pro -o Makefile ) && make -f Makefile prepare_docs
make[2]: Entering directory `/home/jerome/build/dev/qtenginio/src'
cd enginio_client/ && ( test -e Makefile || /home/jerome/build/dev/qtbase/bin/qmake /home/jerome/gerrit/dev/qtenginio/src/enginio_client/enginio_client.pro -o Makefile ) && make -f Makefile prepare_docs
Project MESSAGE: perl -w /home/jerome/gerrit/dev/qtbase/bin/syncqt.pl -module Enginio -version 1.1.0 -outdir /home/jerome/build/dev/qtbase /home/jerome/gerrit/dev/qtenginio
<srcbase> = /home/jerome/gerrit/dev/qtenginio
<outbase> = /home/jerome/build/dev/qtbase
substr outside of string at /home/jerome/gerrit/dev/qtbase/bin/syncqt.pl line 285.
Use of uninitialized value in string eq at /home/jerome/gerrit/dev/qtbase/bin/syncqt.pl line 285.
Project ERROR: Missing CMake tests.
make[2]: *** [sub-enginio_client-prepare_docs] Error 3
make[2]: Leaving directory `/home/jerome/build/dev/qtenginio/src'
make[1]: *** [sub-src-prepare_docs] Error 2
make[1]: Leaving directory `/home/jerome/build/dev/qtenginio'
make: *** [html_docs] Error 2

I am running a prefix+shadow-build 5.4 (dev branch). The doc builds in Jenkins also gets the same error.