diff --git a/src/lib/es6.d.ts b/src/lib/es6.d.ts
index b81a3341a6c..888ded962ef 100644
--- a/src/lib/es6.d.ts
+++ b/src/lib/es6.d.ts
@@ -442,8 +442,8 @@ interface IteratorResult<T> {
 }
 
 interface Iterator<T> {
-    next(): IteratorResult<T>;
-    return?(value?: any): IteratorResult<T>;
+    next(value?: any): IteratorResult<T>;
+    return?(value?: T): IteratorResult<T>;
     throw?(e?: any): IteratorResult<T>;
 }
 
diff --git a/tests/baselines/reference/for-of30.errors.txt b/tests/baselines/reference/for-of30.errors.txt
index 6434b5294d5..bc93a031bf5 100644
--- a/tests/baselines/reference/for-of30.errors.txt
+++ b/tests/baselines/reference/for-of30.errors.txt
@@ -3,7 +3,7 @@ tests/cases/conformance/es6/for-ofStatements/for-of30.ts(1,15): error TS2322: Ty
     Type '() => StringIterator' is not assignable to type '() => Iterator<string>'.
       Type 'StringIterator' is not assignable to type 'Iterator<string>'.
         Types of property 'return' are incompatible.
-          Type 'number' is not assignable to type '(value?: any) => IteratorResult<string>'.
+          Type 'number' is not assignable to type '(value?: string) => IteratorResult<string>'.
 
 
 ==== tests/cases/conformance/es6/for-ofStatements/for-of30.ts (1 errors) ====
@@ -14,7 +14,7 @@ tests/cases/conformance/es6/for-ofStatements/for-of30.ts(1,15): error TS2322: Ty
 !!! error TS2322:     Type '() => StringIterator' is not assignable to type '() => Iterator<string>'.
 !!! error TS2322:       Type 'StringIterator' is not assignable to type 'Iterator<string>'.
 !!! error TS2322:         Types of property 'return' are incompatible.
-!!! error TS2322:           Type 'number' is not assignable to type '(value?: any) => IteratorResult<string>'.
+!!! error TS2322:           Type 'number' is not assignable to type '(value?: string) => IteratorResult<string>'.
     
     class StringIterator {
         next() {
diff --git a/tests/baselines/reference/for-of31.errors.txt b/tests/baselines/reference/for-of31.errors.txt
index 74afff00e99..6d5f6e816be 100644
--- a/tests/baselines/reference/for-of31.errors.txt
+++ b/tests/baselines/reference/for-of31.errors.txt
@@ -3,7 +3,7 @@ tests/cases/conformance/es6/for-ofStatements/for-of31.ts(1,15): error TS2322: Ty
     Type '() => StringIterator' is not assignable to type '() => Iterator<string>'.
       Type 'StringIterator' is not assignable to type 'Iterator<string>'.
         Types of property 'next' are incompatible.
-          Type '() => { value: string; }' is not assignable to type '() => IteratorResult<string>'.
+          Type '() => { value: string; }' is not assignable to type '(value?: any) => IteratorResult<string>'.
             Type '{ value: string; }' is not assignable to type 'IteratorResult<string>'.
               Property 'done' is missing in type '{ value: string; }'.
 
@@ -16,7 +16,7 @@ tests/cases/conformance/es6/for-ofStatements/for-of31.ts(1,15): error TS2322: Ty
 !!! error TS2322:     Type '() => StringIterator' is not assignable to type '() => Iterator<string>'.
 !!! error TS2322:       Type 'StringIterator' is not assignable to type 'Iterator<string>'.
 !!! error TS2322:         Types of property 'next' are incompatible.
-!!! error TS2322:           Type '() => { value: string; }' is not assignable to type '() => IteratorResult<string>'.
+!!! error TS2322:           Type '() => { value: string; }' is not assignable to type '(value?: any) => IteratorResult<string>'.
 !!! error TS2322:             Type '{ value: string; }' is not assignable to type 'IteratorResult<string>'.
 !!! error TS2322:               Property 'done' is missing in type '{ value: string; }'.
     
diff --git a/tests/baselines/reference/iteratorSpreadInArray9.errors.txt b/tests/baselines/reference/iteratorSpreadInArray9.errors.txt
index e0e3d061f16..90b8bdbeb46 100644
--- a/tests/baselines/reference/iteratorSpreadInArray9.errors.txt
+++ b/tests/baselines/reference/iteratorSpreadInArray9.errors.txt
@@ -3,7 +3,7 @@ tests/cases/conformance/es6/spread/iteratorSpreadInArray9.ts(1,17): error TS2322
     Type '() => SymbolIterator' is not assignable to type '() => Iterator<symbol>'.
       Type 'SymbolIterator' is not assignable to type 'Iterator<symbol>'.
         Types of property 'next' are incompatible.
-          Type '() => { value: symbol; }' is not assignable to type '() => IteratorResult<symbol>'.
+          Type '() => { value: symbol; }' is not assignable to type '(value?: any) => IteratorResult<symbol>'.
             Type '{ value: symbol; }' is not assignable to type 'IteratorResult<symbol>'.
               Property 'done' is missing in type '{ value: symbol; }'.
 
@@ -16,7 +16,7 @@ tests/cases/conformance/es6/spread/iteratorSpreadInArray9.ts(1,17): error TS2322
 !!! error TS2322:     Type '() => SymbolIterator' is not assignable to type '() => Iterator<symbol>'.
 !!! error TS2322:       Type 'SymbolIterator' is not assignable to type 'Iterator<symbol>'.
 !!! error TS2322:         Types of property 'next' are incompatible.
-!!! error TS2322:           Type '() => { value: symbol; }' is not assignable to type '() => IteratorResult<symbol>'.
+!!! error TS2322:           Type '() => { value: symbol; }' is not assignable to type '(value?: any) => IteratorResult<symbol>'.
 !!! error TS2322:             Type '{ value: symbol; }' is not assignable to type 'IteratorResult<symbol>'.
 !!! error TS2322:               Property 'done' is missing in type '{ value: symbol; }'.