diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index ccff01a8722..2c457a432c6 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -244,6 +244,9 @@ interface Function {
       */
     bind(this: Function, thisArg: any, ...argArray: any[]): any;
 
+    /** Returns a string representation of a function. */
+    toString(): string;
+
     prototype: any;
     readonly length: number;
 
diff --git a/tests/cases/fourslash/functionTypes.ts b/tests/cases/fourslash/functionTypes.ts
index 8973f73431b..0baccba286e 100644
--- a/tests/cases/fourslash/functionTypes.ts
+++ b/tests/cases/fourslash/functionTypes.ts
@@ -23,7 +23,7 @@
 verify.numberOfErrorsInCurrentFile(0);
 for (var i = 1; i <= 7; i++) {
     goTo.marker('' + i);
-    verify.memberListCount(7);
+    verify.memberListCount(8);
     verify.completionListContains('apply');
     verify.completionListContains('arguments');
     verify.completionListContains('bind');
@@ -31,4 +31,5 @@ for (var i = 1; i <= 7; i++) {
     verify.completionListContains('length');
     verify.completionListContains('caller');
     verify.completionListContains('prototype');
+    verify.completionListContains('toString');
 }