diff --git a/plugin.sh b/plugin.sh
index 68f7eca..ec67383 100755
--- a/plugin.sh
+++ b/plugin.sh
@@ -33,7 +33,7 @@ run_docker_cp() {
   CONTAINER=$(docker -H "$REMOTE_DOCKER_HOST" container ls \
     --format "table {{.ID}},{{.Names}}" \
     | grep "${PLUGIN_SERVICE}" | cut -d',' -f1)
-  if [ -z $CONTAINER ]; then
+  if [ -z "$CONTAINER" ]; then
     echo "ERROR: can't find container for $PLUGIN_SERVICE"
     return 1
   fi