diff --git a/run.sh b/run.sh
index ec894d6..dae271a 100755
--- a/run.sh
+++ b/run.sh
@@ -8,7 +8,7 @@ mkdir -p "$SCRIPT_DIR/build"
 
 echo "Got versions $REV"
 
-if [ -z ${M2_DIRECTORY+x} ]; then
+if [ -z "$M2_DIRECTORY" ]; then
     echo "m2 directory not specified, defaulting to $SCRIPT_DIR/m2"
     M2_DIRECTORY="$SCRIPT_DIR/m2"