diff --git a/arpspoof/rootfs/scripts/99-run.sh b/arpspoof/rootfs/scripts/99-run.sh
index b92412943..189fe4877 100644
--- a/arpspoof/rootfs/scripts/99-run.sh
+++ b/arpspoof/rootfs/scripts/99-run.sh
@@ -1,7 +1,7 @@
 #!/usr/bin/env bashio
 
 # Autodefine if not defined
-if [ -z INTERFACE_NAME ]; then
+if [ ! -z INTERFACE_NAME ]; then
 export INTERFACE_NAME=$(ip route get 8.8.8.8 | sed -nr 's/.*dev ([^\ ]+).*/\1/p')
 bashio::log.blue "Autodetection : INTERFACE_NAME=$INTERFACE_NAME"
 fi