diff --git a/templates/Mapper_Shim.php b/templates/Mapper_Shim.php
index f7c7eec..d9f7636 100644
--- a/templates/Mapper_Shim.php
+++ b/templates/Mapper_Shim.php
@@ -4,6 +4,7 @@
  * This allows all mappers to be serializable with json
  */
 class Mapper_Shim extends Db\SQL\Mapper implements JsonSerializable {
+	#[\ReturnTypeWillChange]
 	public function jsonSerialize() {
 		return $this->cast();
 	}