diff --git a/trytond/model/modelstorage.py b/trytond/model/modelstorage.py
index fa9841be55..56440b7454 100644
--- a/trytond/model/modelstorage.py
+++ b/trytond/model/modelstorage.py
@@ -1412,6 +1412,8 @@ class EvalEnvironment(dict):
         if item in self._model._fields:
             value = getattr(self._record, item)
             if isinstance(value, Model):
+                if self._model._fields[item]._type == 'reference':
+                    return str(value)
                 return value.id
             elif isinstance(value, (list, tuple)):
                 return [r.id for r in value]