diff --git a/.dd4hep-ci.d/compile_and_test.sh b/.dd4hep-ci.d/compile_and_test.sh
index af4e08d47f57df52c2088bec0eba6e581cc1e320..a5dc21cf2ac0d2b875bd6aed57b73b86ec917418 100755
--- a/.dd4hep-ci.d/compile_and_test.sh
+++ b/.dd4hep-ci.d/compile_and_test.sh
@@ -5,6 +5,7 @@ source /DD4hep/.dd4hep-ci.d/init_x86_64.sh
 cd /DD4hep
 mkdir build
 cd build
+unset CPATH  # causes implicit includes to be added in cmake, preventing setting some system libraries to -isystem
 cmake -GNinja -DDD4HEP_USE_GEANT4=ON -DBoost_NO_BOOST_CMAKE=ON -DDD4HEP_USE_LCIO=ON -DBUILD_TESTING=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_FLAGS="-fdiagnostics-color=always -Werror" -DCMAKE_CXX_STANDARD=${STANDARD} .. && \
 ninja && \
 ninja install && \