diff --git a/.github/changelog/fix-3332-migration-cpt-order b/.github/changelog/fix-3332-migration-cpt-order
new file mode 100644
index 0000000000..571a5b5af3
--- /dev/null
+++ b/.github/changelog/fix-3332-migration-cpt-order
@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Fixed a PHP notice that could appear when ActivityPub migration ran before custom post types were registered.
diff --git a/includes/class-migration.php b/includes/class-migration.php
index 39f1d5fef9..b4c4887719 100644
--- a/includes/class-migration.php
+++ b/includes/class-migration.php
@@ -26,7 +26,7 @@ class Migration {
 	 * Initialize the class, registering WordPress hooks.
 	 */
 	public static function init() {
-		self::maybe_migrate();
+		\add_action( 'init', array( self::class, 'maybe_migrate' ), 20 );
 
 		Scheduler::register_async_batch_callback( 'activitypub_migrate_from_0_17', array( self::class, 'migrate_from_0_17' ) );
 		Scheduler::register_async_batch_callback( 'activitypub_update_comment_counts', array( self::class, 'update_comment_counts' ) );
diff --git a/tests/phpunit/tests/includes/class-test-migration.php b/tests/phpunit/tests/includes/class-test-migration.php
index 5bfaf2962d..7f8bc886db 100644
--- a/tests/phpunit/tests/includes/class-test-migration.php
+++ b/tests/phpunit/tests/includes/class-test-migration.php
@@ -633,6 +633,19 @@ public function mock_webfinger() {
 		);
 	}
 
+	/**
+	 * Test migration is deferred until post types are registered.
+	 */
+	public function test_init_defers_migration_until_after_post_types() {
+		\remove_action( 'init', array( Migration::class, 'maybe_migrate' ), 20 );
+		$this->setExpectedIncorrectUsage( 'Activitypub\\Scheduler::register_async_batch_callback' );
+
+		Migration::init();
+
+		$this->assertSame( 20, \has_action( 'init', array( Migration::class, 'maybe_migrate' ) ) );
+		\remove_action( 'init', array( Migration::class, 'maybe_migrate' ), 20 );
+	}
+
 	/**
 	 * Test add_default_extra_field.
 	 */