diff --git a/SendBird.d.ts b/SendBird.d.ts
index 4c26631..bcdc487 100644
--- a/SendBird.d.ts
+++ b/SendBird.d.ts
@@ -16,7 +16,7 @@ interface SendBirdStatic {
 }
 
 declare namespace SendBird {
-  interface SendBirdError {
+  interface SendBirdError extends Error {
     code: number;
     message: string;
   }
@@ -354,9 +354,9 @@ declare namespace SendBird {
     isIdentical(target: BaseMessageInstance): boolean;
     isOpenChannel(): boolean;
     isGroupChannel(): boolean;
-    isUserMessage(): boolean;
-    isFileMessage(): boolean;
-    isAdminMessage(): boolean;
+    isUserMessage(): this is UserMessage;
+    isFileMessage(): this is FileMessage;
+    isAdminMessage(): this is AdminMessage;
     serialize(): Object;
     getMetaArraysByKeys(keys: Array<string>): Array<MessageMetaArray>;
     applyReactionEvent(event: ReactionEvent): void;
@@ -616,8 +616,8 @@ declare namespace SendBird {
     isEphemeral: boolean;
     createdAt: number;
 
-    isGroupChannel(): boolean;
-    isOpenChannel(): boolean;
+    isGroupChannel(): this is GroupChannel;
+    isOpenChannel(): this is OpenChannel;
     serialize(): Object;
 
     /* DEPRECATED */