diff --git a/OpenApiPhp/Util.php b/OpenApiPhp/Util.php
index dc0db90e8..8a3537f56 100644
--- a/OpenApiPhp/Util.php
+++ b/OpenApiPhp/Util.php
@@ -424,7 +424,7 @@ private static function mergeFromArray(OA\AbstractAnnotation $annotation, array
             if ('$ref' === $propertyName) {
                 $propertyName = 'ref';
             }
-            if (!\in_array($propertyName, $done, true)) {
+            if (array_key_exists($propertyName, $defaults) && !\in_array($propertyName, $done, true)) {
                 self::mergeProperty($annotation, $propertyName, $value, $defaults[$propertyName], $overwrite);
             }
         }