diff --git a/types/toposort/index.d.ts b/types/toposort/index.d.ts
index c2c9940c3a..ed94597140 100644
--- a/types/toposort/index.d.ts
+++ b/types/toposort/index.d.ts
@@ -1,7 +1,8 @@
 // Type definitions for toposort 2.0
 // Project: https://github.com/marcelklehr/toposort
 // Definitions by: Daniel Byrne <https://github.com/danwbyrne>
+//                 Prokop Simek <https://github.com/prokopsimek>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
-declare function toposort(graph: ReadonlyArray<[string, string]>): ReadonlyArray<string>;
+declare function toposort(graph: Array<[string, string | undefined]>): string[];
 export = toposort;
diff --git a/types/toposort/toposort-tests.ts b/types/toposort/toposort-tests.ts
index 84f554033b..9dcd4fa10d 100644
--- a/types/toposort/toposort-tests.ts
+++ b/types/toposort/toposort-tests.ts
@@ -1,10 +1,11 @@
 import toposort = require('toposort');
 
-const testGraph: ReadonlyArray<[string, string]> = [
-    ["string1", "string2"],
-    ["string2", "string3"],
-    ["string3", "string1"]
+const testGraph: Array<[string, string | undefined]> = [
+    ['string1', 'string2'],
+    ['string2', 'string3'],
+    ['string3', 'string1'],
+    ['string4', undefined],
 ];
 
-// $ExpectType ReadonlyArray<string>
+// $ExpectType string[]
 toposort(testGraph);