diff --git a/flow/Set.js b/flow/Set.js
index 97557b499..7293d3b08 100644
--- a/flow/Set.js
+++ b/flow/Set.js
@@ -20,6 +20,7 @@ declare module "Set" {
   // messages about "Using Set instead of Set".
   declare class SetPolyfill<T> {
     @@iterator(): Iterator<T>;
+    constructor(iterable: ?Iterable<T>): void;
     add(value: T): SetPolyfill<T>;
     clear(): void;
     delete(value: T): boolean;