diff --git a/projects/plugins/jetpack/changelog/fix-nl-715-post-access-level-non-string-guard b/projects/plugins/jetpack/changelog/fix-nl-715-post-access-level-non-string-guard
new file mode 100644
index 000000000000..9beb57004022
--- /dev/null
+++ b/projects/plugins/jetpack/changelog/fix-nl-715-post-access-level-non-string-guard
@@ -0,0 +1,4 @@
+Significance: patch
+Type: bugfix
+
+Newsletter: default the post access level to "everybody" when the stored meta is not a string, and sanitize non-string writes, so corrupt values can no longer cause a fatal error when rendering a post.
diff --git a/projects/plugins/jetpack/extensions/blocks/subscriptions/subscriptions.php b/projects/plugins/jetpack/extensions/blocks/subscriptions/subscriptions.php
index 4e329cc5a1c3..9fa785be2b56 100644
--- a/projects/plugins/jetpack/extensions/blocks/subscriptions/subscriptions.php
+++ b/projects/plugins/jetpack/extensions/blocks/subscriptions/subscriptions.php
@@ -79,10 +79,18 @@ function register_block() {
 		'post',
 		META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS,
 		array(
-			'show_in_rest'  => true,
-			'single'        => true,
-			'type'          => 'string',
-			'auth_callback' => function () {
+			'show_in_rest'      => true,
+			'single'            => true,
+			'type'              => 'string',
+			// The REST schema already rejects non-strings, but non-REST writers
+			// (importers, XML-RPC, WP-CLI, direct update_post_meta, sync) only pass
+			// through sanitize_meta(). Coerce anything that isn't a string to ''
+			// ("everybody") so a corrupt value can't be persisted and later fatal the
+			// strict string-typed access checks.
+			'sanitize_callback' => function ( $value ) {
+				return is_string( $value ) ? $value : '';
+			},
+			'auth_callback'     => function () {
 				return wp_get_current_user()->has_cap( 'edit_posts' );
 			},
 		)
diff --git a/projects/plugins/jetpack/modules/memberships/class-jetpack-memberships.php b/projects/plugins/jetpack/modules/memberships/class-jetpack-memberships.php
index 17436d438f65..b25a209c3d4b 100644
--- a/projects/plugins/jetpack/modules/memberships/class-jetpack-memberships.php
+++ b/projects/plugins/jetpack/modules/memberships/class-jetpack-memberships.php
@@ -700,7 +700,13 @@ public static function get_post_access_level( $post_id = null ) {
 		}
 
 		$post_access_level = get_post_meta( $post_id, self::$post_access_level_meta_name, true );
-		if ( empty( $post_access_level ) ) {
+		// Defaults to "everybody" when unset, and also when the stored value is not a
+		// string. Corrupt rows (e.g. a serialized array like a:1:{i:0;s:0:"";}) can be
+		// persisted by non-REST write paths, and an array flows unchanged into the
+		// strict string-typed `earn_user_has_access` callback on WPCOM, fataling the
+		// render. Coercing here keeps this canonical accessor's documented string
+		// contract regardless of how the meta was written.
+		if ( empty( $post_access_level ) || ! is_string( $post_access_level ) ) {
 			$post_access_level = Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_EVERYBODY;
 		}
 
diff --git a/projects/plugins/jetpack/tests/php/modules/subscriptions/Jetpack_Subscriptions_Access_Level_Meta_Test.php b/projects/plugins/jetpack/tests/php/modules/subscriptions/Jetpack_Subscriptions_Access_Level_Meta_Test.php
new file mode 100644
index 000000000000..37b0f63250b3
--- /dev/null
+++ b/projects/plugins/jetpack/tests/php/modules/subscriptions/Jetpack_Subscriptions_Access_Level_Meta_Test.php
@@ -0,0 +1,193 @@
+<?php
+/**
+ * Tests for the post-level newsletter access meta (`_jetpack_newsletter_access`),
+ * covering NL-715: corrupt (non-string) values must not reach the strict
+ * string-typed access checks and fatal the render.
+ *
+ * Two layers are exercised:
+ *  - Read side: Jetpack_Memberships::get_post_access_level() coerces any
+ *    non-string stored value to the "everybody" default.
+ *  - Write side: the register_post_meta() sanitize_callback coerces a non-string
+ *    write to '' so the corrupt value can't be persisted via sanitize_meta()
+ *    write paths in the first place.
+ *
+ * @package automattic/jetpack
+ */
+
+use Automattic\Jetpack\Extensions\Premium_Content\Subscription_Service\Abstract_Token_Subscription_Service;
+use PHPUnit\Framework\Attributes\CoversClass;
+use function Automattic\Jetpack\Extensions\Subscriptions\register_block as register_subscription_block;
+use const Automattic\Jetpack\Extensions\Subscriptions\META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS;
+
+require_once JETPACK__PLUGIN_DIR . 'extensions/blocks/premium-content/_inc/subscription-service/include.php';
+require_once JETPACK__PLUGIN_DIR . 'modules/memberships/class-jetpack-memberships.php';
+require_once JETPACK__PLUGIN_DIR . 'extensions/blocks/subscriptions/subscriptions.php';
+
+/**
+ * @covers Jetpack_Memberships
+ */
+#[CoversClass( Jetpack_Memberships::class )]
+class Jetpack_Subscriptions_Access_Level_Meta_Test extends WP_UnitTestCase {
+	use \Automattic\Jetpack\PHPUnit\WP_UnitTestCase_Fix;
+
+	/**
+	 * Reset the static access-level cache between tests so a value cached under a
+	 * blog/post key in one test cannot leak into another.
+	 *
+	 * @return void
+	 */
+	public function set_up() {
+		parent::set_up();
+		Jetpack_Memberships::clear_post_access_level_cache();
+	}
+
+	/**
+	 * Writes a raw value straight into the postmeta table, bypassing
+	 * update_post_meta()'s sanitization, to faithfully reproduce the corrupt rows
+	 * that non-REST write paths persisted in production (e.g. a:1:{i:0;s:0:"";}).
+	 *
+	 * @param int   $post_id The post to attach the meta to.
+	 * @param mixed $value   The raw (already-serializable) value to store.
+	 * @return void
+	 */
+	private function force_corrupt_access_meta( $post_id, $value ) {
+		global $wpdb;
+		$wpdb->insert(
+			$wpdb->postmeta,
+			array(
+				'post_id'    => $post_id,
+				'meta_key'   => META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS,
+				'meta_value' => maybe_serialize( $value ),
+			)
+		);
+		wp_cache_delete( $post_id, 'post_meta' );
+		Jetpack_Memberships::clear_post_access_level_cache();
+	}
+
+	/**
+	 * The exact corrupt shape seen in production — an array containing one empty
+	 * string — must be treated as "everybody" rather than flowing through as an
+	 * array and fataling the strict string-typed `earn_user_has_access` callback.
+	 *
+	 * This is the regression test for NL-715: it fails before the guard (returns
+	 * the array) and passes after (returns the EVERYBODY default).
+	 *
+	 * @return void
+	 */
+	public function test_array_access_level_meta_defaults_to_everybody() {
+		$post_id = self::factory()->post->create();
+		$this->force_corrupt_access_meta( $post_id, array( '' ) );
+
+		// Sanity check: the corrupt array really is persisted and is not empty().
+		$raw = get_post_meta( $post_id, META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS, true );
+		$this->assertIsArray( $raw );
+		$this->assertNotEmpty( $raw );
+
+		$this->assertSame(
+			Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_EVERYBODY,
+			Jetpack_Memberships::get_post_access_level( $post_id )
+		);
+	}
+
+	/**
+	 * A multi-value array (another corrupt shape a non-REST writer could persist)
+	 * is likewise coerced to the everybody default rather than returned as an array.
+	 *
+	 * @return void
+	 */
+	public function test_multi_value_array_access_level_meta_defaults_to_everybody() {
+		$post_id = self::factory()->post->create();
+		$this->force_corrupt_access_meta(
+			$post_id,
+			array(
+				Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_PAID_SUBSCRIBERS,
+				Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_SUBSCRIBERS,
+			)
+		);
+
+		$this->assertSame(
+			Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_EVERYBODY,
+			Jetpack_Memberships::get_post_access_level( $post_id )
+		);
+	}
+
+	/**
+	 * A valid string access level is returned unchanged — the guard must not
+	 * regress the normal path.
+	 *
+	 * @return void
+	 */
+	public function test_valid_string_access_level_meta_is_returned_unchanged() {
+		$post_id = self::factory()->post->create();
+		update_post_meta(
+			$post_id,
+			META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS,
+			Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_SUBSCRIBERS
+		);
+		Jetpack_Memberships::clear_post_access_level_cache();
+
+		$this->assertSame(
+			Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_SUBSCRIBERS,
+			Jetpack_Memberships::get_post_access_level( $post_id )
+		);
+	}
+
+	/**
+	 * An unset access level falls back to everybody, as before.
+	 *
+	 * @return void
+	 */
+	public function test_missing_access_level_meta_defaults_to_everybody() {
+		$post_id = self::factory()->post->create();
+
+		$this->assertSame(
+			Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_EVERYBODY,
+			Jetpack_Memberships::get_post_access_level( $post_id )
+		);
+	}
+
+	/**
+	 * Write-side defense (NL-715): once the meta is registered, an array written
+	 * through the normal update_post_meta() path is coerced to '' by the
+	 * sanitize_callback, so the corrupt value can never be persisted in the first
+	 * place via writers that route through sanitize_meta().
+	 *
+	 * @return void
+	 */
+	public function test_sanitize_callback_coerces_array_write_to_empty_string() {
+		// Activate the subscriptions module so register_block() actually registers
+		// the meta (it returns early when the module is inactive).
+		Jetpack_Options::update_option( 'active_modules', array( 'subscriptions' ) );
+		register_subscription_block();
+
+		$post_id = self::factory()->post->create();
+		update_post_meta( $post_id, META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS, array( '' ) );
+
+		$this->assertSame(
+			'',
+			get_post_meta( $post_id, META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS, true )
+		);
+	}
+
+	/**
+	 * The same write-side defense must leave a legitimate string write untouched.
+	 *
+	 * @return void
+	 */
+	public function test_sanitize_callback_preserves_valid_string_write() {
+		Jetpack_Options::update_option( 'active_modules', array( 'subscriptions' ) );
+		register_subscription_block();
+
+		$post_id = self::factory()->post->create();
+		update_post_meta(
+			$post_id,
+			META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS,
+			Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_SUBSCRIBERS
+		);
+
+		$this->assertSame(
+			Abstract_Token_Subscription_Service::POST_ACCESS_LEVEL_SUBSCRIBERS,
+			get_post_meta( $post_id, META_NAME_FOR_POST_LEVEL_ACCESS_SETTINGS, true )
+		);
+	}
+}