diff --git a/should/should.d.ts b/should/should.d.ts
index ce2743dfd8..b0e6054bde 100644
--- a/should/should.d.ts
+++ b/should/should.d.ts
@@ -40,6 +40,7 @@ interface ShouldAssertion {
   Function(): ShouldAssertion;
   Date(): ShouldAssertion;
   Class(): ShouldAssertion;
+  Undefined(): ShouldAssertion;
   generator(): ShouldAssertion;
   iterable(): ShouldAssertion;
   iterator(): ShouldAssertion;