diff --git a/src/REST/keywords.py b/src/REST/keywords.py
index cfeb519..adad430 100644
--- a/src/REST/keywords.py
+++ b/src/REST/keywords.py
@@ -1321,6 +1321,7 @@ def rest_instances(self, file_path=None, sort_keys=False):
             self.instances,
             ensure_ascii=False,
             indent=4,
+            default=vars,
             separators=(",", ": "),
             sort_keys=sort_keys,
         )