diff --git a/python/ray/autoscaler/autoscaler.py b/python/ray/autoscaler/autoscaler.py
index e97e56477..fb1c6389b 100644
--- a/python/ray/autoscaler/autoscaler.py
+++ b/python/ray/autoscaler/autoscaler.py
@@ -503,14 +503,16 @@ def check_required(config, schema):
 def check_extraneous(config, schema):
     """Make sure all items of config are in schema"""
     if type(config) is not dict:
-        raise ValueError("Config is not a dictionary")
+        raise ValueError("Config {} is not a dictionary".format(config))
     for k in config:
         if k not in schema:
             raise ValueError(
                 "Unexpected config key `{}` not in {}".format(
                     k, list(schema.keys())))
         v, kreq = schema[k]
-        if isinstance(v, type):
+        if v is None:
+            continue
+        elif isinstance(v, type):
             if not isinstance(config[k], v):
                 raise ValueError(
                     "Config key `{}` has wrong type {}, expected {}".format(
@@ -522,7 +524,7 @@ def check_extraneous(config, schema):
 def validate_config(config, schema=CLUSTER_CONFIG_SCHEMA):
     """Required Dicts indicate that no extra fields can be introduced."""
     if type(config) is not dict:
-        raise ValueError("Config is not a dictionary")
+        raise ValueError("Config {} is not a dictionary".format(config))
 
     check_required(config, schema)
     check_extraneous(config, schema)