diff --git a/type-definitions/Immutable.d.ts b/type-definitions/Immutable.d.ts
index baae41d011..05f572b4d8 100644
--- a/type-definitions/Immutable.d.ts
+++ b/type-definitions/Immutable.d.ts
@@ -3878,7 +3878,8 @@ declare module Immutable {
      * In case the `Collection` is empty returns the optional default
      * value if provided, if no default value is provided returns undefined.
      */
-    first<NSV>(notSetValue?: NSV): V | NSV;
+    first<NSV>(): V | undefined;
+    first<NSV>(notSetValue: NSV): V | NSV;
 
     /**
      * In case the `Collection` is not empty returns the last element of the
@@ -3886,7 +3887,8 @@ declare module Immutable {
      * In case the `Collection` is empty returns the optional default
      * value if provided, if no default value is provided returns undefined.
      */
-    last<NSV>(notSetValue?: NSV): V | NSV;
+    last<NSV>(): V | undefined;
+    last<NSV>(notSetValue: NSV): V | NSV;
 
     // Reading deep values
 
diff --git a/type-definitions/immutable.js.flow b/type-definitions/immutable.js.flow
index 2b37f41437..c4aa6d191a 100644
--- a/type-definitions/immutable.js.flow
+++ b/type-definitions/immutable.js.flow
@@ -63,8 +63,10 @@ declare class _Collection<K, +V> implements ValueObject {
   has(key: K): boolean;
   includes(value: V): boolean;
   contains(value: V): boolean;
-  first<NSV>(notSetValue?: NSV): V | NSV;
-  last<NSV>(notSetValue?: NSV): V | NSV;
+  first(): V | undefined;
+  first<NSV>(notSetValue: NSV): V | NSV;
+  last(): V | undefined;
+  last<NSV>(notSetValue: NSV): V | NSV;
 
   hasIn(keyPath: Iterable<mixed>): boolean;