diff --git a/baselines/dom.generated.d.ts b/baselines/dom.generated.d.ts
index 980c65030..8b339ce06 100644
--- a/baselines/dom.generated.d.ts
+++ b/baselines/dom.generated.d.ts
@@ -5896,10 +5896,10 @@ interface GlobalEventHandlers {
      */
     ontimeupdate: ((this: GlobalEventHandlers, ev: Event) => any) | null;
     ontoggle: ((this: GlobalEventHandlers, ev: Event) => any) | null;
-    ontouchcancel?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchend?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchmove?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
-    ontouchstart?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null;
+    ontouchcancel?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null | undefined;
+    ontouchend?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null | undefined;
+    ontouchmove?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null | undefined;
+    ontouchstart?: ((this: GlobalEventHandlers, ev: TouchEvent) => any) | null | undefined;
     ontransitioncancel: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
     ontransitionend: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
     ontransitionrun: ((this: GlobalEventHandlers, ev: TransitionEvent) => any) | null;
diff --git a/src/build/emitter.ts b/src/build/emitter.ts
index 3768ae5e2..80cfdaccf 100644
--- a/src/build/emitter.ts
+++ b/src/build/emitter.ts
@@ -791,7 +791,7 @@ export function emitWebIdl(
       } else {
         pType = convertDomTypeToTsType(p);
       }
-      if (p.optional && prefix) {
+      if (p.optional) {
         pType += " | undefined";
       }
       const optionalModifier = !p.optional || prefix ? "" : "?";