diff --git a/src/Traits/HttpBodyTrait.php b/src/Traits/HttpBodyTrait.php
index b95b230..2b28a9f 100644
--- a/src/Traits/HttpBodyTrait.php
+++ b/src/Traits/HttpBodyTrait.php
@@ -45,6 +45,9 @@ public function withAddedBody(string $body): object
      */
     public function withBodyJson($body): object
     {
+        if (!$this->hasHeader('Content-Type')) {
+            $this->withHeader('Content-Type', 'application/json');
+        }
         return $this->withBody(json_encode($body));
     }