diff --git a/config.yaml b/config.yaml
index 133587ca..e895ba80 100644
--- a/config.yaml
+++ b/config.yaml
@@ -60,8 +60,8 @@ options:
       A space-separated list of NICs that we want phy-nic-mtu applied to.
   phy-nic-mtu:
     type: int
-    default: 0
+    default:
     description: |
       To improve network performance of VM, sometimes we should keep VM MTU as
       1500 and use charm to modify MTU of tunnel nic more than 1500 (e.g. 1546
-      for GRE). A value of zero means no mtu will be set/modified.
+      for GRE).