diff --git a/types/memoize-one/index.d.ts b/types/memoize-one/index.d.ts
index 71f785e73c..e8ec195875 100644
--- a/types/memoize-one/index.d.ts
+++ b/types/memoize-one/index.d.ts
@@ -1,9 +1,9 @@
-// Type definitions for memoize-one 3.1
+// Type definitions for memoize-one 4.1
 // Project: https://github.com/alexreardon/memoize-one#readme
 // Definitions by: Karol Majewski <https://github.com/karol-majewski>, Frank Li <https://github.com/franklixuefei>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 declare function memoizeOne<T extends (...args: any[]) => any>(resultFn: T, isEqual?: EqualityFn): T;
-export type EqualityFn = (a: any, b: any) => boolean;
+export type EqualityFn = (a: any, b: any, index: number) => boolean;
 
 export default memoizeOne;
diff --git a/types/memoize-one/memoize-one-tests.ts b/types/memoize-one/memoize-one-tests.ts
index 4feddf6e57..a9c565b500 100644
--- a/types/memoize-one/memoize-one-tests.ts
+++ b/types/memoize-one/memoize-one-tests.ts
@@ -3,6 +3,7 @@ import memoizeOne, { EqualityFn } from 'memoize-one';
 declare function add(a: number, b: number): number ;
 declare function lousyEqualityFn(a: any, b: any): boolean;
 declare function strictEqualityFn<T>(a: T, b: T): boolean;
+declare function equalityFnWithIndex<T>(a: T, b: T, index: number): boolean;
 
 /**
  * Accepts a second argument.
@@ -10,17 +11,18 @@ declare function strictEqualityFn<T>(a: T, b: T): boolean;
 memoizeOne(add); // $ExpectType (a: number, b: number) => number
 memoizeOne(add, lousyEqualityFn); // $ExpectType (a: number, b: number) => number
 memoizeOne(add, strictEqualityFn); // $ExpectType (a: number, b: number) => number
+memoizeOne(add, equalityFnWithIndex); // $ExpectType (a: number, b: number) => number
 
 /**
- * The second argument can be, but doesn't have to be stritly typed.
+ * The second argument can be, but doesn't have to be strictly typed.
  */
 memoizeOne(add, (a, b) => a === b); // $ExpectType (a: number, b: number) => number
 memoizeOne(add, (a: string, b: string) => a === b); // $ExpectType (a: number, b: number) => number
 
 /**
- * Function passed as the second argument accepts exactly two arguments.
+ * Function passed as the second argument accepts no more than three arguments.
  */
-memoizeOne(add, (a: number, b: number, c: number) => a === b || c); // $ExpectError
+memoizeOne(add, (a: number, b: number, c: number, d: number) => Boolean(a &&  b && c && d)); // $ExpectError
 
 /**
  * Function passed as the second argument returns a boolean.