diff --git a/baselines/dom.generated.d.ts b/baselines/dom.generated.d.ts
index 980c65030..c604af4dd 100644
--- a/baselines/dom.generated.d.ts
+++ b/baselines/dom.generated.d.ts
@@ -17230,7 +17230,12 @@ interface Window extends EventTarget, AnimationFrameProvider, GlobalEventHandler
     readonly pageXOffset: number;
     /** @deprecated This is a legacy alias of `scrollY`. */
     readonly pageYOffset: number;
-    readonly parent: WindowProxy | null;
+    /**
+     * Refers to either the parent WindowProxy, or itself.
+     *
+     * It can rarely be null e.g. for contentWindow of an iframe that is already removed from the parent.
+     */
+    readonly parent: WindowProxy;
     /**
      * Returns true if the personal bar is visible; otherwise, returns false.
      */
@@ -18294,7 +18299,12 @@ declare var outerWidth: number;
 declare var pageXOffset: number;
 /** @deprecated This is a legacy alias of `scrollY`. */
 declare var pageYOffset: number;
-declare var parent: WindowProxy | null;
+/**
+ * Refers to either the parent WindowProxy, or itself.
+ *
+ * It can rarely be null e.g. for contentWindow of an iframe that is already removed from the parent.
+ */
+declare var parent: WindowProxy;
 /**
  * Returns true if the personal bar is visible; otherwise, returns false.
  */
diff --git a/inputfiles/comments.json b/inputfiles/comments.json
index 7920d6f86..01b6c0fb5 100644
--- a/inputfiles/comments.json
+++ b/inputfiles/comments.json
@@ -1796,6 +1796,15 @@
                         }
                     }
                 }
+            },
+            "Window": {
+                "properties": {
+                    "property": {
+                        "parent": {
+                            "comment" : "/**\n * Refers to either the parent WindowProxy, or itself.\n *\n * It can rarely be null e.g. for contentWindow of an iframe that is already removed from the parent.\n */"
+                        }
+                    }
+                }
             }
         }
     }
diff --git a/inputfiles/overridingTypes.jsonc b/inputfiles/overridingTypes.jsonc
index 0d552dd97..5bf3cc46e 100644
--- a/inputfiles/overridingTypes.jsonc
+++ b/inputfiles/overridingTypes.jsonc
@@ -500,6 +500,9 @@
                         },
                         "window": {
                             "overrideType": "Window & typeof globalThis"
+                        },
+                        "parent": {
+                            "nullable": false
                         }
                     }
                 },