diff --git a/types/fetch.io/fetch.io-tests.ts b/types/fetch.io/fetch.io-tests.ts
index 538da8e74b..67c2612fdc 100644
--- a/types/fetch.io/fetch.io-tests.ts
+++ b/types/fetch.io/fetch.io-tests.ts
@@ -57,3 +57,8 @@ request
   .post('')
   .send({})
   .json();
+
+request
+  .post('')
+  .append('key', 'value')
+  .append({key: 'value'});
diff --git a/types/fetch.io/index.d.ts b/types/fetch.io/index.d.ts
index 8258cbf8da..fabbbc4b52 100644
--- a/types/fetch.io/index.d.ts
+++ b/types/fetch.io/index.d.ts
@@ -1,4 +1,4 @@
-// Type definitions for fetch.io 3.1
+// Type definitions for fetch.io 4.0
 // Project: https://github.com/haoxins/fetch.io
 // Definitions by: newraina <https://github.com/newraina>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
@@ -95,7 +95,9 @@ export class Request {
   /**
    * ppend formData
    */
-  append(key: string, value: string): this;
+  append(key: string, value: any): this;
+
+  append(object: {[key: string]: any}): this;
 
   /**
    * Get Response directly