diff --git a/v1/index.d.ts b/v1/index.d.ts
index 0fbc6eb..a206c64 100644
--- a/v1/index.d.ts
+++ b/v1/index.d.ts
@@ -33,7 +33,7 @@ export class Cloudevent {
   public data(data: any): Cloudevent;
   public getData(): any;
 
-  public addExtension(name: string, value: any): void;
+  public addExtension(name: string, value: any): Cloudevent;
 
   public getExtensions(): Map<string, any>;
 }