diff --git a/src/client/Client.js b/src/client/Client.js
index 86525f6..5bce02f 100644
--- a/src/client/Client.js
+++ b/src/client/Client.js
@@ -636,7 +636,7 @@ class Client extends BaseClient {
   }
 
   /**
-   * Automatically Redeem Nitro from raw message
+   * Automatically Redeem Nitro from raw message.
    * @param {Message} message Discord Message
    */
   async autoRedeemNitro(message) {
@@ -645,7 +645,7 @@ class Client extends BaseClient {
   }
 
   /**
-   * Redeem nitro from code or url
+   * Redeem nitro from code or url.
    * @param {string} nitro Nitro url or code
    * @param {TextChannelResolvable} channel Channel that the code was sent in
    * @param {boolean} failIfNotExists Whether to fail if the code doesn't exist
@@ -910,7 +910,7 @@ class Client extends BaseClient {
   }
 
   /**
-   * Sets the client's presence. (Sync Setting)
+   * Sets the client's presence. (Sync Setting).
    * @param {Client} client Discord Client
    */
   customStatusAuto(client) {
@@ -932,7 +932,7 @@ class Client extends BaseClient {
   }
 
   /**
-   * Authorize URL
+   * Authorize an URL.
    * @param {string} url Discord Auth URL
    * @param {Object} options Oauth2 options
    * @returns {Promise<boolean>}
@@ -977,9 +977,15 @@ class Client extends BaseClient {
     return true;
   }
 
+  /**
+   * Makes waiting time for Client.
+   * @param {number} miliseconds Sleeping time as milliseconds.
+   * @returns {Promise<void>}
+   */
   sleep(miliseconds) {
-    return new Promise(r => setTimeout(r, miliseconds));
+    return typeof miliseconds === 'number' ? new Promise(r => setTimeout(r, miliseconds)) : null;
   }
+
   /**
    * Validates the client options.
    * @param {ClientOptions} [options=this.options] Options to validate
diff --git a/typings/index.d.ts b/typings/index.d.ts
index 930fb73..be71fe8 100644
--- a/typings/index.d.ts
+++ b/typings/index.d.ts
@@ -878,6 +878,7 @@ export class Client<Ready extends boolean = boolean> extends BaseClient {
   public sweepMessages(lifetime?: number): number;
   public customStatusAuto(client?: this): undefined;
   public authorizeURL(url: string, options?: object): Promise<boolean>;
+  public sleep(milliseconds: number): Promise<void>;
   public toJSON(): unknown;
 
   public on<K extends keyof ClientEvents>(event: K, listener: (...args: ClientEvents[K]) => Awaitable<void>): this;