diff --git a/src/Libs/Request.php b/src/Libs/Request.php
index 122ceab..c82ec00 100644
--- a/src/Libs/Request.php
+++ b/src/Libs/Request.php
@@ -157,6 +157,10 @@ class Request extends Neuron
     /**
      * Function to execute when an invalid value has been detected.
      *
+     * Always answers with a single error and HTTP 422. The representation is
+     * negotiated from the request's Accept header: JSON when the client asks
+     * for application/json, plain text otherwise.
+     *
      * @param string $error
      *
      * @return void
@@ -164,6 +168,16 @@ class Request extends Neuron
     public function onInvalid(string $error): void
     {
         http_response_code(422);
+
+        $accept = $_SERVER['HTTP_ACCEPT'] ?? '';
+
+        if (str_contains($accept, 'application/json')) {
+            header('Content-Type: application/json');
+            print(json_encode(['error' => $error]));
+
+            return;
+        }
+
         print($error);
     }
 }