diff -u b/core/modules/migrate/src/Annotation/MigrateDestination.php b/core/modules/migrate/src/Annotation/MigrateDestination.php
--- b/core/modules/migrate/src/Annotation/MigrateDestination.php
+++ b/core/modules/migrate/src/Annotation/MigrateDestination.php
@@ -46,12 +46,11 @@
   /**
    * Identifies the system handling the data the destination plugin will write.
    *
-   * This can be any type, and the destination plugin itself determines how the
-   * value is used. For example, Migrate Drupal's destination plugins expect
-   * destination_module to be the name of a module that must be installed on the
-   * destination.
+   * The destination plugin itself determines how the value is used. For
+   * example, Migrate Drupal's destination plugins expect destination_module to
+   * be the name of a module that must be installed on the destination.
    *
-   * @var mixed
+   * @var string
    */
   public $destination_module;
 
diff -u b/core/modules/migrate/src/Annotation/MigrateSource.php b/core/modules/migrate/src/Annotation/MigrateSource.php
--- b/core/modules/migrate/src/Annotation/MigrateSource.php
+++ b/core/modules/migrate/src/Annotation/MigrateSource.php
@@ -43,14 +43,13 @@
   /**
    * Identifies the system providing the data the source plugin will read.
    *
-   * This can be any type, and the source plugin itself determines how the value
-   * is used. For example, Migrate Drupal's source plugins expect source_module
-   * to be the name of a module that must be installed and enabled in the source
-   * database.
+   * The source plugin itself determines how the value is used. For example,
+   * Migrate Drupal's source plugins expect source_module to be the name of a
+   * module that must be installed and enabled in the source database.
    *
    * @see \Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase::checkRequirements
    *
-   * @var mixed
+   * @var string
    */
   public $source_module;