diff --git a/src/deque.d.ts b/src/deque.d.ts
index 02b1de8..f0a5c74 100644
--- a/src/deque.d.ts
+++ b/src/deque.d.ts
@@ -2,10 +2,10 @@ export class Deque<T> {
   constructor(elements?: T[]);
   pushFront(element: T): Deque<T>;
   pushBack(element: T): Deque<T>;
-  popFront(): T;
-  popBack(): T;
-  front(): T;
-  back(): T;
+  popFront(): T | null;
+  popBack(): T | null;
+  front(): T | null;
+  back(): T | null;
   toArray(): T[];
   isEmpty(): boolean;
   size(): number;