diff --git a/types/parse-prefer-header/index.d.ts b/types/parse-prefer-header/index.d.ts
index 40d5424a8e..9cda4ecc15 100644
--- a/types/parse-prefer-header/index.d.ts
+++ b/types/parse-prefer-header/index.d.ts
@@ -1,8 +1,8 @@
 // Type definitions for parse-prefer-header 1.0
 // Project: https://github.com/ppaskaris/node-parse-prefer-header
-// Definitions by: Vincenzo Chianese <https://github.com/XVincentX>
+// Definitions by: Vincenzo Chianese <https://github.com/XVincentX>, Marcell Toth <https://github.com/marcelltoth>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
-declare function parsePreferHeader(preferHeader: string | string[]): { [key: string]: string | boolean };
+declare function parsePreferHeader(preferHeader: string | ReadonlyArray<string> | null | undefined): { [key: string]: string | true };
 
 export = parsePreferHeader;
diff --git a/types/parse-prefer-header/parse-prefer-header-tests.ts b/types/parse-prefer-header/parse-prefer-header-tests.ts
index 20a1eb7efd..10493ecca7 100644
--- a/types/parse-prefer-header/parse-prefer-header-tests.ts
+++ b/types/parse-prefer-header/parse-prefer-header-tests.ts
@@ -1,4 +1,11 @@
 import parsePreferHeader = require('parse-prefer-header');
 
-parsePreferHeader(['respond-async, wait=100', 'handling=lenient']);
+const testArray = ['respond-async, wait=100', 'handling=lenient'];
+
+const readonlyTestArray: ReadonlyArray<string> = testArray;
+
+parsePreferHeader(testArray);
+parsePreferHeader(readonlyTestArray);
 parsePreferHeader('');
+parsePreferHeader(null);
+parsePreferHeader(undefined);