diff --git a/elements/apt-preferences/extra-data.d/99-set-apt-package-pins b/elements/apt-preferences/extra-data.d/99-set-apt-package-pins
index 537d2457..8a62c2aa 100755
--- a/elements/apt-preferences/extra-data.d/99-set-apt-package-pins
+++ b/elements/apt-preferences/extra-data.d/99-set-apt-package-pins
@@ -18,7 +18,7 @@ set -eu
 set -o pipefail
 
 # exit directly if DIB_DPKG_MANIFEST is not defined properly
-if [ -z "$DIB_DPKG_MANIFEST" ]; then
+if [ -z "${DIB_DPKG_MANIFEST:-}" ]; then
     echo "DIB_DPKG_MANIFEST must be set to the location of a manifest file you wish to use"
     exit 0
 elif [ ! -f "$DIB_DPKG_MANIFEST" -o ! -s "$DIB_DPKG_MANIFEST" ]; then
diff --git a/elements/apt-sources/extra-data.d/99-override-default-apt-sources b/elements/apt-sources/extra-data.d/99-override-default-apt-sources
index 0c8ae185..d0d928db 100755
--- a/elements/apt-sources/extra-data.d/99-override-default-apt-sources
+++ b/elements/apt-sources/extra-data.d/99-override-default-apt-sources
@@ -5,9 +5,9 @@ set -eu
 set -o pipefail
 
 # exit directly if DIB_APT_SOURCES is not defined properly
-if [ -z "$DIB_APT_SOURCES" ] ; then
+if [ -z "${DIB_APT_SOURCES:-}" ] ; then
     echo "DIB_APT_SOURCES must be set to the location of a sources.list file you wish to use"
-    exit 1
+    exit 0
 elif [ ! -f "$DIB_APT_SOURCES" -o ! -s "$DIB_APT_SOURCES" ] ; then
     echo "$DIB_APT_SOURCES  is not a valid sources.list file."
     echo "You should assign proper sources.list file in DIB_APT_SOURCES"