diff --git a/zipline/transforms/utils.py b/zipline/transforms/utils.py
index 59f40cb8..d1f88007 100644
--- a/zipline/transforms/utils.py
+++ b/zipline/transforms/utils.py
@@ -121,7 +121,8 @@ class StatefulTransform(object):
         # Normal object instantiation.
         else:
             self.state = tnfm_class(*args, **kwargs)
-
+        # save the window_length of the state for external access.
+        self.window_length = self.state.window_length
         # Create the string associated with this generator's output.
         self.namestring = tnfm_class.__name__ + hash_args(*args, **kwargs)