diff --git a/sofort/cfgtest.sh b/sofort/cfgtest.sh
index b4e5686..caa000a 100644
--- a/sofort/cfgtest.sh
+++ b/sofort/cfgtest.sh
@@ -114,7 +114,7 @@ cfgtest_header_presence()
 			| sed -e 's/\./_/g' -e 's@/@_@g'  \
 			| tr "[:lower:]" "[:upper:]")
 
-	if [ -z $cfgtest_internal_unit_test ]; then
+	if [ -z ${cfgtest_internal_unit_test:-} ]; then
 		cfgtest_cflags_append "$mb_internal_str"
 	else
 		cfgtest_makevar_append "$mb_internal_str"
@@ -135,7 +135,7 @@ cfgtest_header_absence()
 			| sed -e 's/\./_/g' -e 's@/@_@g'  \
 			| tr "[:lower:]" "[:upper:]")
 
-	if [ -z $cfgtest_internal_unit_test ]; then
+	if [ -z ${cfgtest_internal_unit_test:-} ]; then
 		cfgtest_cflags_append "$mb_internal_str"
 	else
 		cfgtest_makevar_append "$mb_internal_str"
@@ -162,7 +162,7 @@ cfgtest_interface_presence()
 			| sed -e 's/\./_/g'             \
 			| tr "[:lower:]" "[:upper:]")
 
-	if [ -z $cfgtest_internal_unit_test ]; then
+	if [ -z ${cfgtest_internal_unit_test:-} ]; then
 		cfgtest_cflags_append "$mb_internal_str"
 	else
 		cfgtest_makevar_append "$mb_internal_str"
@@ -198,7 +198,7 @@ cfgtest_decl_presence()
 			| sed -e 's/\./_/g'                  \
 			| tr "[:lower:]" "[:upper:]")
 
-	if [ -z $cfgtest_internal_unit_test ]; then
+	if [ -z ${cfgtest_internal_unit_test:-} ]; then
 		cfgtest_cflags_append "$mb_internal_str"
 	else
 		cfgtest_makevar_append "$mb_internal_str"