Don't return null error

lib/Message.php

@@ -3,10 +3,12 @@
 
 namespace AdvancedJsonRpc;
 
+use JsonSerializable;
+
 /**
  * Base message
  */
-abstract class Message
+abstract class Message implements JsonSerializable
 {
     /**
      * A String specifying the version of the JSON-RPC protocol. MUST be exactly "2.0".
@@ -41,4 +43,9 @@ public function __toString(): string
     {
         return json_encode($this);
     }
+
+    public function jsonSerialize()
+    {
+        return ['jsonrpc' => $this->jsonrpc];
+    }
 }

lib/Response.php

@@ -3,11 +3,13 @@
 
 namespace AdvancedJsonRpc;
 
+use JsonSerializable;
+
 /**
  * When a rpc call is made, the Server MUST reply with a Response, except for in the case of Notifications. The Response
  * is expressed as a single JSON Object, with the following members:
  */
-class Response extends Message
+class Response extends Message implements JsonSerializable
 {
     /**
      * This member is REQUIRED. It MUST be the same as the value of the id member in the Request Object. If there was an
@@ -55,4 +57,16 @@ public function __construct($id, $result, ResponseError $error = null)
         $this->result = $result;
         $this->error = $error;
     }
+
+    public function jsonSerialize()
+    {
+        $json = parent::jsonSerialize();
+        $json['id'] = $this->id;
+        if (isset($this->error)) {
+            $json['error'] = $this->error;
+        } else {
+            $json['result'] = $this->result;
+        }
+        return $json;
+    }
 }