diff --git a/tests/installed/run.sh b/tests/installed/run.sh
index 38a9d769..8c7fe4d8 100755
--- a/tests/installed/run.sh
+++ b/tests/installed/run.sh
@@ -4,7 +4,7 @@ set -xeuo pipefail
 
 dn=$(dirname $0)
 for tn in ${dn}/itest-*.sh; do
-    if [ -n "${TESTS+ }" ]; then
+    if [ -n "${TESTS:-}" ]; then
       tbn=$(basename "$tn" .sh)
       tbn=" ${tbn#itest-} "
       if [[ " $TESTS " != *$tbn* ]]; then