Cluster's one Server down getting error

For a test purpose I created one cluster with 4 servers. We would like to use the cluster as memcached feature. Everything works with out problem when all the servers up an running. But when I shut one of the servers we get randomly “SERVER_ERROR proxy write to downstream” error the ip address belongs to the shut one.

We set 50 different keys with a php page to one of three servers and when we checked the keys from web console there were 45 keys and 5 keys were not set by the errors.

Somehow Moxi proxy does not understand the server failure and still send the request to all servers including the shut one.

Is there any way reduce the moxiproxy’s heartbeat settings or some tricks for the errors?