diff --git a/tools/build_uec.sh b/tools/build_uec.sh
index 89d4661..f420d6b 100755
--- a/tools/build_uec.sh
+++ b/tools/build_uec.sh
@@ -168,7 +168,7 @@ EOF
 # (re)start a metadata service
 (
   pid=`lsof -iTCP@192.168.$GUEST_NETWORK.1:4567 -n | awk '{print $2}' | tail -1`
-  [ ! -e $pid ] || kill -9 $pid
+  [ -z "$pid" ] || kill -9 $pid
 )
 cd $vm_dir/uec
 python meta.py 192.168.$GUEST_NETWORK.1:4567 &