diff --git a/baselines/dom.generated.d.ts b/baselines/dom.generated.d.ts
index a7878878a..9bd71bd0e 100644
--- a/baselines/dom.generated.d.ts
+++ b/baselines/dom.generated.d.ts
@@ -2491,7 +2491,7 @@ declare var DOMStringList: {
 }
 
 interface DOMStringMap {
-    [name: string]: string;
+    [name: string]: string | undefined;
 }
 
 declare var DOMStringMap: {
diff --git a/inputfiles/overridingTypes.json b/inputfiles/overridingTypes.json
index 7bc701061..1e904294c 100644
--- a/inputfiles/overridingTypes.json
+++ b/inputfiles/overridingTypes.json
@@ -980,5 +980,10 @@
         "readonly": true,
         "name": "previousElementSibling",
         "type": "Element | null"
+    },
+    {
+        "kind": "indexer",
+        "interface": "DOMStringMap",
+        "signatures": ["[name: string]: string | undefined"]
     }
  ]