diff --git a/sleep.ts b/sleep.ts
index 2cfd4c4..8c5a9f2 100644
--- a/sleep.ts
+++ b/sleep.ts
@@ -1,3 +1,3 @@
 export function sleep(milliseconds: number) {
-  return new Promise((resolve) => setTimeout(resolve, milliseconds));
+  return new Promise<void>((resolve) => setTimeout(resolve, milliseconds));
 }