Help to find out why error appear

I got phones MAC’s addresses from router in Item Wifiusers, but when Item Wifiusers is updated WARN appear in logs:

2016-10-06 19:45:19.793 [INFO ] [org.openhab.model.script.Tets ] - Wifiusers changed 11:6E:1F:6C:C6:BB 11:FE:34:FC:65:DD
2016-10-06 19:45:20.304 [WARN ] [ore.internal.items.ItemUpdater] - InstantiationException on org.openhab.core.library.types.StringType

What is wrong in my rules?

String    Mobile_A            "My phone is at home [MAP(]"    <network>    (gMobiles)
rule "Wifiusers" 
    Item Wifiusers changed 
    if (Wifiusers.state.toString.contains("11:6E:1F:6C:C6:BB")) {
    } else { Mobile_A.postUpdate(OFF) }

You’re trying to set a String item to ON, did you mean for this item to be a Switch?

Yes, you right. It must be Switch.
Thank you.