Error: [$compile:nonassign] Expression ‘undefined’ used with directive ‘myFacebook’ is non-assignable

My solution was harder to find out here, but easier to implement. I had to change it to the equivalent of (Note that the question mark makes the attribute optional. Prior to 1.5 this apparently wasn’t required).

scope: {
    permissions: '@',
    myFriends: '=?friends'
}