diff --git a/composer.json b/composer.json
index c71c1e58..ddaa9b54 100644
--- a/composer.json
+++ b/composer.json
@@ -13,7 +13,7 @@
         "nikic/php-parser": "< 4.12.0",
         "php-stubs/generator": "^0.8.3",
         "phpdocumentor/reflection-docblock": "^5.3",
-        "phpstan/phpstan": "^1.2"
+        "phpstan/phpstan": "^1.9"
     },
     "suggest": {
         "paragonie/sodium_compat": "Pure PHP implementation of libsodium",
diff --git a/functionMap.php b/functionMap.php
index 1a505e16..c44b4c2a 100644
--- a/functionMap.php
+++ b/functionMap.php
@@ -65,4 +65,5 @@
     'WP_Block_List::offsetGet' => ['WP_Block|null', 'offset'=>'int'],
     'WP_Block_List::offsetSet' => ['void', 'offset'=>'int|null'],
     'WP_Block_List::offsetUnset' => ['void', 'offset'=>'int'],
+    'is_wp_error' => ['bool', '@phpstan-assert-if-true'=>'\WP_Error $thing']
 ];
diff --git a/wordpress-stubs.php b/wordpress-stubs.php
index 9157cb6a..956ed348 100644
--- a/wordpress-stubs.php
+++ b/wordpress-stubs.php
@@ -110284,6 +110284,8 @@ function wp_doing_cron()
      *
      * @param mixed $thing The variable to check.
      * @return bool Whether the variable is an instance of WP_Error.
+     * @phpstan-assert-if-true \WP_Error $thing
+     * @phpstan-return bool
      */
     function is_wp_error($thing)
     {