"HV000030: No validator could be found for type" for string mapped enumeration in Hibernate

javax.validation.UnexpectedTypeException: HV000030: No validator could be found for type my.types.Day.

public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY 
}

import javax.persistence.Enumerated;
import javax.persistence.EnumType;

@Entity
public class Week ... {
    @Enumerated(EnumType.STRING)
    ...
    private Day day;
}