diff --git a/debian/dump1090-mutability.config b/debian/dump1090-mutability.config
index ced85dd..e0b8e15 100644
--- a/debian/dump1090-mutability.config
+++ b/debian/dump1090-mutability.config
@@ -184,9 +184,9 @@ is_port_number() {
 
 is_port_list() {
   for port in $(echo $1 | sed 's@,@ @g'); do
-    if ! is_port_number "$port"; then return 0; fi
+    if ! is_port_number "$port"; then return 1; fi
   done
-  return 1
+  return 0
 }
 
 db_input high $NAME/auto-start || true