diff --git a/system/API/ResponseTrait.php b/system/API/ResponseTrait.php
index ece7effb0a96..ae2183e55ccc 100644
--- a/system/API/ResponseTrait.php
+++ b/system/API/ResponseTrait.php
@@ -358,7 +358,7 @@ protected function format($data = null)
      */
     protected function setResponseFormat(?string $format = null)
     {
-        $this->format = strtolower($format);
+        $this->format = ($format === null) ? null : strtolower($format);
 
         return $this;
     }