diff --git a/src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php b/src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
index f98d1ad57e..caee7204ea 100644
--- a/src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
+++ b/src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
@@ -519,6 +519,16 @@ abstract class WP_REST_Controller {
 	public function get_fields_for_response( $request ) {
 		$schema = $this->get_item_schema();
 		$fields = isset( $schema['properties'] ) ? array_keys( $schema['properties'] ) : array();
+
+		$additional_fields = $this->get_additional_fields();
+		foreach ( $additional_fields as $field_name => $field_options ) {
+			// For back-compat, include any field with an empty schema
+			// because it won't be present in $this->get_item_schema().
+			if ( is_null( $field_options['schema'] ) ) {
+				$fields[] = $field_name;
+			}
+		}
+
 		if ( ! isset( $request['_fields'] ) ) {
 			return $fields;
 		}
diff --git a/tests/phpunit/tests/rest-api/rest-posts-controller.php b/tests/phpunit/tests/rest-api/rest-posts-controller.php
index 10f6a6eebc..ff97d2648d 100644
--- a/tests/phpunit/tests/rest-api/rest-posts-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-posts-controller.php
@@ -3365,6 +3365,38 @@ class WP_Test_REST_Posts_Controller extends WP_Test_REST_Post_Type_Controller_Te
 		$wp_rest_additional_fields = array();
 	}
 
+	/**
+	 * @ticket 45220
+	 */
+	public function test_get_additional_field_registration_null_schema() {
+		register_rest_field( 'post', 'my_custom_int', array(
+			'schema'          => null,
+			'get_callback'    => array( $this, 'additional_field_get_callback' ),
+			'update_callback' => null,
+		) );
+		$post_id = $this->factory->post->create();
+
+		// 'my_custom_int' should appear because ?_fields= isn't set.
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/posts/' . $post_id );
+		$response = $this->server->dispatch( $request );
+		$this->assertArrayHasKey( 'my_custom_int', $response->data );
+
+		// 'my_custom_int' should appear because it's present in ?_fields=.
+		$request = new WP_REST_Request( 'GET', '/wp/v2/posts/' . $post_id );
+		$request->set_param( '_fields', 'title,my_custom_int' );
+		$response = $this->server->dispatch( $request );
+		$this->assertArrayHasKey( 'my_custom_int', $response->data );
+
+		// 'my_custom_int' should not appear because it's not present in ?_fields=.
+		$request = new WP_REST_Request( 'GET', '/wp/v2/posts/' . $post_id );
+		$request->set_param( '_fields', 'title' );
+		$response = $this->server->dispatch( $request );
+		$this->assertArrayNotHasKey( 'my_custom_int', $response->data );
+
+		global $wp_rest_additional_fields;
+		$wp_rest_additional_fields = array();
+	}
+
 	public function test_additional_field_update_errors() {
 		$schema = array(
 			'type'        => 'integer',