diff --git a/zipline/utils/events.py b/zipline/utils/events.py
index 562301f0..0247e4b8 100644
--- a/zipline/utils/events.py
+++ b/zipline/utils/events.py
@@ -175,7 +175,9 @@ class EventRule(six.with_metaclass(ABCMeta)):
     """
     An event rule checks a datetime and sees if it should trigger.
     """
-    env = TradingEnvironment.instance()
+    @property
+    def env(self):
+        return TradingEnvironment.instance()
 
     @abstractmethod
     def should_trigger(self, dt):