diff --git a/tlsproxy b/tlsproxy
index 344c3ac..4f9febe 100755
--- a/tlsproxy
+++ b/tlsproxy
@@ -7,7 +7,7 @@ set -xe
 cd $(dirname $0)
 
 CLUSTER="$1"
-if [ ! -d "cluster/$CLUSTER" ]; then
+if [ ! -d "cluster/$CLUSTER" ] || [ -z "$CLUSTER" ]; then
 	echo "Usage: $0 <cluster name>"
 	echo "The cluster name must be the name of a subdirectory of cluster/"
 	exit 1