Link monitor problem

Hello,

 I'm trying to set up a backup WAN connection. In one location it works fine, but WAN connections there are configured as VLANs and connected through a switch.

 When they are connected directly to physical ports and untagged I receive a gateway error.

Here is my config:

set srcintf wan1 set server 8.8.4.4 set gateway-ip xxx.xxx.xxx.177 set source-ip xxx.xxx.xxx.178 set interval 5 set failtime 5 set recoverytime 5 set update-cascade-interface enable set update-static-route enable set status enable end

I also created static routes with a different priority and separate policies for each connection.

When I try to enter this config I receive:

Gateway is not unique for the same interface.

object set operator error, -7 discard the setting

Command fail. Return code -7

WAN settings are: 

manual IP and the static route has a static gateway.

I have a few VPN tunnels (policy and interface based) under this physical interface.

What am I doing wrong?

EDIT: I'm using Fortigate 60E with latest OS (FortiOS v6.0.5 build0268 (GA))