AttributeError when custom agent check run by datadog agent

gistfile1.txt

# output in /var/log/datadog/collector.log
2016-09-03 00:25:46 UTC | ERROR | dd.collector | checks.statsrelay_check(__init__.py:767) | Check 'statsrelay_check' instance #0 failed
Traceback (most recent call last):
  File "/opt/datadog-agent/agent/checks/__init__.py", line 750, in run
    self.check(copy.deepcopy(instance))
  File "/etc/dd-agent/checks.d/statsrelay_check.py", line 14, in check
    metrics = fetch_metrics(host, port)
  File "/etc/dd-agent/checks.d/statsrelay_check.py", line 30, in fetch_metrics
    t.write('status\n\n')
  File "/opt/datadog-agent/embedded/lib/python2.7/telnetlib.py", line 283, in write
    self.sock.sendall(buffer)
AttributeError: 'NoneType' object has no attribute 'sendall'

statsrelay_check.py

#!/usr/bin/env python

from telnetlib import Telnet
from checks import AgentCheck

CONFIG_FILE = '/etc/dd-agent/conf.d/statsrelay_check.yaml'


class StatsRelayCheck(AgentCheck):
    def check(self, instance):
        host = instance.get('host')
        port = instance.get('port')
        metrics = fetch_metrics(host, port)
        prefix = self.init_config.get('prefix')
        for k, v in metrics.iteritems():
            if prefix:
                metric_name = prefix + k
            type_ = v[0]
            value = v[-1]
            getattr(self, type_)(metric=metric_name, value=value)

def fetch_metrics(host, port):
    type_overrides = {
        'boolean': 'gauge',
        'timestamp': 'gauge'
    }
    metrics = dict()
    t = Telnet(host, port, 5)
    t.write('status\n\n')
    output = t.read_all()
    t.close()

    for l in output.splitlines():
        if l:
            scope, metric, type_, value  = l.rstrip().split()
            if type_ in type_overrides.keys():
                type_ = type_overrides.get(type_)
            scope = scope.replace('-', '_').replace('.', '_').replace(':','_')
            metric_name = '{}.{}.{}'.format(metric_prefix, scope, metric)
            metric_val = value
            metrics[metric_name] = [type_, value]
    return metrics

if __name__ == '__main__':
    check, instances = StatsRelayCheck.from_yaml(CONFIG_FILE)
    for instance in instances:
        check.check(instance)
        print check.get_metrics()