With an empty collection, run this:

db.Food.update(
    { "names" : { "$all" : [ "corn"]}},
    { "$addToSet" : { "names" : "corn"}},
    { upsert: true });

Results in this error:
Cannot apply $addToSet to a non-array field. Field named 'names' has a non-array type String in the document INVALID-MUTABLE-ELEMENT

Tested in 2.6.3
It worked fine in 2.4.10