Google_pubsub_subscription : dead_letter_policy not recognized

i am trying to add dead_letter_policy to my subscription but i get this error:

on main.tf line 127, in resource “google_pubsub_subscription” “example”:
127: dead_letter_policy {

Blocks of type “dead_letter_policy” are not expected here.

i am using the sample code in repository:
resource “google_pubsub_topic” “example” {
name = “example-topic”
}

resource “google_pubsub_topic” “example_dead_letter” {
name = “example-topic-dead-letter”
}

resource “google_pubsub_subscription” “example” {
name = “example-subscription”
topic = google_pubsub_topic.example.name

dead_letter_policy {
dead_letter_topic = google_pubsub_topic.example_dead_letter.id
max_delivery_attempts = 10
}
}

terraform version 0.14.3