diff --git a/docker-rsync/agent-start b/docker-rsync/agent-start
index b63892b..1d3c425 100755
--- a/docker-rsync/agent-start
+++ b/docker-rsync/agent-start
@@ -6,7 +6,7 @@ FOLDER=${1:-default}
 STORE_PATH="/tmp/ssh-agent/$FOLDER"
 mkdir -p "$STORE_PATH"
 
-if [ -z "$SSH_AGENT_PID" ]; then
+if [ -z "${SSH_AGENT_PID:-}" ]; then
 	if [ -f "$STORE_PATH/id" ]; then
 		SSH_AGENT_PID=$(cat "$STORE_PATH/id")
 		export SSH_AGENT_PID