diff --git a/.jenkins/lsu/env-clang-17.sh b/.jenkins/lsu/env-clang-17.sh
index ac4ab8e51d9..07db8d953f9 100644
--- a/.jenkins/lsu/env-clang-17.sh
+++ b/.jenkins/lsu/env-clang-17.sh
@@ -15,7 +15,7 @@ module load openmpi
 export HPXRUN_RUNWRAPPER=srun
 export CXX_STD="20"
 export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:$PKG_CONFIG_PATH
-export CXXFLAGS="-fopenmp -Wno-pass-failed ${CXXFLAGS}"
+export CXXFLAGS="-fopenmp -Wno-pass-failed ${CXXFLAGS:-}"
 
 configure_extra_options+=" -DCMAKE_BUILD_TYPE=${build_type}"
 configure_extra_options+=" -DHPX_WITH_CXX_STANDARD=${CXX_STD}"
diff --git a/.jenkins/lsu/env-gcc-13.sh b/.jenkins/lsu/env-gcc-13.sh
index 42a783a8d21..160b7aa9b8e 100644
--- a/.jenkins/lsu/env-gcc-13.sh
+++ b/.jenkins/lsu/env-gcc-13.sh
@@ -15,7 +15,7 @@ module load openmpi
 export HPXRUN_RUNWRAPPER=srun
 export CXX_STD="20"
 export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:$PKG_CONFIG_PATH
-export CXXFLAGS="-fopenmp ${CXXFLAGS}"
+export CXXFLAGS="-fopenmp ${CXXFLAGS:-}"
 
 configure_extra_options+=" -DHPX_WITH_CXX_STANDARD=${CXX_STD}"
 configure_extra_options+=" -DHPX_WITH_MALLOC=system"