diff --git a/types/shallowequal/index.d.ts b/types/shallowequal/index.d.ts
index 95b91d6de5..4d111eb332 100644
--- a/types/shallowequal/index.d.ts
+++ b/types/shallowequal/index.d.ts
@@ -2,14 +2,24 @@
 // Project: https://github.com/dashed/shallowequal
 // Definitions by: Sean Kelley <https://github.com/seansfkelley>
 //                 BendingBender <https://github.com/BendingBender>
+//                 Arnd Issler <https://github.com/arndissler>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.3
 
 declare function shallowEqual<TCtx = any>(
     objA: any,
     objB: any,
-    compare?: (this: TCtx, objA: any, objB: any, indexOrKey?: number | string) => boolean | void,
+    customizer?: shallowEqual.Customizer<TCtx>,
     compareContext?: TCtx
 ): boolean;
 
+declare namespace shallowEqual {
+    type Customizer<T = any> = (
+        this: T,
+        objA: any,
+        objB: any,
+        indexOrKey?: number | string
+    ) => boolean | void;
+}
+
 export = shallowEqual;
diff --git a/types/shallowequal/shallowequal-tests.ts b/types/shallowequal/shallowequal-tests.ts
index efa5b546a5..6d30381472 100644
--- a/types/shallowequal/shallowequal-tests.ts
+++ b/types/shallowequal/shallowequal-tests.ts
@@ -12,13 +12,34 @@ shallowEqual(a, b, (a, b, indexOrKey) => {
 
     return false;
 });
-shallowEqual(a, b, () => {}); // $ExpectType boolean
+shallowEqual(a, b, () => undefined); // $ExpectType boolean
 // $ExpectType boolean
 shallowEqual(
     a,
     b,
     function() {
         this; // $ExpectType { foo: string; }
+        return undefined;
     },
     { foo: 'bar' }
 );
+
+interface Foo {
+    foo: string;
+    bar: number;
+}
+
+const c: Foo = { foo: 'foo', bar: 0 };
+const d: Foo = { foo: 'baz', bar: 1 };
+
+const undefinedCustomizer: shallowEqual.Customizer<Foo> = () => undefined;
+
+const customizer: shallowEqual.Customizer<Foo> = function(a: any, b: any) {
+    this; // $ExpectType Foo
+    a; // $ExpectType any
+    b; // $ExpectType any
+    return undefined;
+};
+
+shallowEqual(c, d, undefinedCustomizer); // $ExpectType boolean
+shallowEqual(c, d, customizer); // $ExpectType boolean