diff --git a/fluent/handler.py b/fluent/handler.py
index f549f91..e2ccfac 100644
--- a/fluent/handler.py
+++ b/fluent/handler.py
@@ -142,7 +142,7 @@ def _format_by_exclusion(self, record):
         data = {}
         for key, value in record.__dict__.items():
             if key not in self._exc_attrs:
-                data[key] = value
+                data[key] = str(value)
         return data
 
     def _format_by_dict(self, record):