diff --git a/tools/jenkins/include/scripts/pkg/gnutls.sh b/tools/jenkins/include/scripts/pkg/gnutls.sh
index 2da9798eb5..d850cb6cb7 100644
--- a/tools/jenkins/include/scripts/pkg/gnutls.sh
+++ b/tools/jenkins/include/scripts/pkg/gnutls.sh
@@ -4,7 +4,7 @@
 # see http://www.linuxfromscratch.org/blfs/view/cvs/postlfs/gnutls.html
 GNUTLS_VERSION=3.6.13
 GNUTLS_VERSION_MICRO= #.1
-if [ "${CENTOS}" = 6 ] && [ -z "${DTS}" ]; then
+if [ "${CENTOS:-0}" = 6 ] && [ -z "${DTS+x}" ]; then
     GNUTLS_VERSION=3.6.8
     GNUTLS_VERSION_MICRO=
     # 3.6.9 and later fail on CentOS6 (probably needs an updated binutils/gas) with:
diff --git a/tools/jenkins/include/scripts/pkg/imagemagick6.sh b/tools/jenkins/include/scripts/pkg/imagemagick6.sh
index 9422defe1b..6e60253e43 100644
--- a/tools/jenkins/include/scripts/pkg/imagemagick6.sh
+++ b/tools/jenkins/include/scripts/pkg/imagemagick6.sh
@@ -3,7 +3,7 @@
 # Install ImageMagick6
 # see http://www.linuxfromscratch.org/blfs/view/cvs/general/imagemagick6.html
 MAGICK_VERSION=6.9.11-6
-if [ "${CENTOS}" = 6 ] && [ -z "${DTS}" ]; then
+if [ "${CENTOS:-0}" = 6 ] && [ -z "${DTS+x}" ]; then
     MAGICK_VERSION=6.9.10-78 # 6.9.10-79 and later fail to compile on CentOS6 with "undefined reference to `aligned_alloc'"
 fi
 MAGICK_VERSION_SHORT=${MAGICK_VERSION%-*}
diff --git a/tools/jenkins/include/scripts/pkg/imagemagick7.sh b/tools/jenkins/include/scripts/pkg/imagemagick7.sh
index c7ddf07f74..0bc76e166d 100644
--- a/tools/jenkins/include/scripts/pkg/imagemagick7.sh
+++ b/tools/jenkins/include/scripts/pkg/imagemagick7.sh
@@ -3,7 +3,7 @@
 # install ImageMagick7
 # see http://www.linuxfromscratch.org/blfs/view/cvs/general/imagemagick.html
 MAGICK7_VERSION=7.0.10-6
-if [ "${CENTOS}" = 6 ] && [ -z "${DTS}" ]; then
+if [ "${CENTOS:-0}" = 6 ] && [ -z "${DTS+x}" ]; then
     MAGICK7_VERSION=7.0.9-8 # 7.0.9-9 (probably) and later fail to compile on CentOS6 with "undefined reference to `aligned_alloc'"
 fi
 MAGICK7_VERSION_SHORT=${MAGICK7_VERSION%-*}