diff --git a/.circleci/config.yml b/.circleci/config.yml
index 326a058..b63cb1e 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -2,7 +2,7 @@ version: '2'
 jobs:
   build:
     docker:
-      - image: ubuntu:yakkety
+      - image: ubuntu:xenial
     working_directory: ~/code
     steps:
       - setup_remote_docker
diff --git a/elasticsearch_collectd.py b/elasticsearch_collectd.py
index 05f8b02..b223925 100755
--- a/elasticsearch_collectd.py
+++ b/elasticsearch_collectd.py
@@ -1019,6 +1019,10 @@ def detect_es_master(self):
         cluster_state = self.fetch_url(self.es_url_scheme + "://" +
                                        self.es_host + ":" + str(self.es_port) +
                                        "/_cluster/state/master_node")
+        if cluster_state is None:
+            self.es_current_master = False
+            return
+
         if self.es_current_master is False \
            and cluster_state['master_node'] == self.node_id:
             self.es_current_master = True