Karabiner: How to swap FN and CONTROL_R

I'm using a CM Storm keyboard and I want to swap FN and CONTROL_R. Here is my private.xml

<item>
        <name>Swap FN and Ctrl in ISO keyboard</name>
        <identifier>private.Swap_fn_right_Ctrl_in_iso_keyboard</identifier>
        <device_only>DeviceVendor::CM_STORM, DeviceProduct::MY_CM_STORM_KEYBOARD</device_only>
        <autogen>__KeyToKey__ KeyCode::FN, KeyCode::CONTROL_R</autogen>     
        <autogen>__KeyToKey__ KeyCode::CONTROL_R, KeyCode::FN</autogen>     

    </item>

Since FN and CONTROL_R are KeyModifier, I substituted KeyCode with KeyModifier but it didn't work either.

Also I'm sure there is no problem with my device_onlytag, because currently I have other items which work perfectly fine.

Is there any way to just swap modifier keys?