From the IETF Draft:

5.5.1.1. Valid values

The value of this keyword MUST be an array. This array MUST have at
least one element. Elements in the array MUST be unique.

Elements in the array MAY be of any type, including null.

5.5.1.2. Conditions for successful validation

An instance validates successfully against this keyword if its value
is equal to one of the elements in this keyword's array value.