diff --git a/python/ray/rllib/models/catalog.py b/python/ray/rllib/models/catalog.py
index 776773552..ce91742c3 100644
--- a/python/ray/rllib/models/catalog.py
+++ b/python/ray/rllib/models/catalog.py
@@ -147,7 +147,7 @@ class ModelCatalog(object):
         elif isinstance(action_space, gym.spaces.multi_discrete.MultiDiscrete):
             if torch:
                 raise NotImplementedError
-            return MultiCategorical, sum(action_space.nvec)
+            return MultiCategorical, int(sum(action_space.nvec))
 
         raise NotImplementedError("Unsupported args: {} {}".format(
             action_space, dist_type))