ActiveRecord::RecordNotUnique (PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_users_on_unlock_token"

create_table "users", force: true do |t|
    t.string   "email",                                               default: "",    null: false
    t.string   "encrypted_password",                                  default: "",    null: false
    t.string   "screen_name"
    t.string   "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer  "sign_in_count",                                       default: 0
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.string   "current_sign_in_ip"
    t.string   "last_sign_in_ip"
    t.string   "confirmation_token"
    t.datetime "confirmed_at"
    t.datetime "confirmation_sent_at"
    t.integer  "failed_attempts",                                     default: 0
    t.string   "unlock_token"
    t.datetime "locked_at"
end

SQL (10.5ms)  UPDATE "users" SET "unlock_token" = $1, "updated_at" = $2 WHERE "users"."id" = 7  [["unlock_token", ""], ["updated_at", Mon, 24 Feb 2014 02:59:58 UTC +00:00]]
PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_users_on_unlock_token"
DETAIL:  Key (unlock_token)=() already exists.