diff --git a/dbus/trunk/30-dbus b/dbus/trunk/30-dbus
index 42b3a5f..96a5182 100644
--- a/dbus/trunk/30-dbus
+++ b/dbus/trunk/30-dbus
@@ -1,6 +1,6 @@
 #!/bin/bash
 
 # launches a session dbus instance
-if [ -z "$DBUS_SESSION_BUS_ADDRESS" ] && type dbus-launch >/dev/null; then
+if [ -z "${DBUS_SESSION_BUS_ADDRESS-}" ] && type dbus-launch >/dev/null; then
   eval $(dbus-launch --sh-syntax --exit-with-session)
 fi