diff --git a/src/wuttaweb/forms/schema.py b/src/wuttaweb/forms/schema.py
index 0dabf64..3d0e08b 100644
--- a/src/wuttaweb/forms/schema.py
+++ b/src/wuttaweb/forms/schema.py
@@ -250,12 +250,13 @@ class ObjectRef(colander.SchemaType):
             *args,
             **kwargs,
     ):
+        # nb. allow session injection for tests
+        self.session = kwargs.pop('session', Session())
         super().__init__(*args, **kwargs)
         self.request = request
         self.config = self.request.wutta_config
         self.app = self.config.get_app()
         self.model_instance = None
-        self.session = Session()
 
         if empty_option:
             if empty_option is True: