diff --git a/python/ray/tune/registry.py b/python/ray/tune/registry.py
index 5ffa4e416..250e8b8b2 100644
--- a/python/ray/tune/registry.py
+++ b/python/ray/tune/registry.py
@@ -79,11 +79,11 @@ def register_env(name, env_creator):
 
     Args:
         name (str): Name to register.
-        env_creator (obj): Function that creates an env.
+        env_creator (obj): Callable that creates an env.
     """
 
-    if not isinstance(env_creator, FunctionType):
-        raise TypeError("Second argument must be a function.", env_creator)
+    if not callable(env_creator):
+        raise TypeError("Second argument must be callable.", env_creator)
     _global_registry.register(ENV_CREATOR, name, env_creator)