diff --git a/tools/build_kvm.sh b/tools/build_kvm.sh
index cc7d121..cc63ba2 100755
--- a/tools/build_kvm.sh
+++ b/tools/build_kvm.sh
@@ -20,7 +20,7 @@ TOOLS_DIR=$(cd $(dirname "$0") && pwd)
 TOP_DIR=$TOOLS_DIR/..
 
 # Configure the root password of the vm
-ROOT_PASSWORD=${ROOT_PASSWORD:password}
+ROOT_PASSWORD=${ROOT_PASSWORD:-password}
 
 # Where to store files and instances
 KVMSTACK_DIR=${KVMSTACK_DIR:-/opt/kvmstack}