buffer_size field value is negative number from "internal" plugin

[ghost]

System Info: CentOS 7.2.1511 & 4 cores, 12GB RAM

Version: Telegraf v1.2.1

Conf File:
metric_buffer_limit = 500000
there is 8 influxdb outputs configured

[[inputs.internal]]
  ## If true, collect telegraf memory stats.
  # collect_memstats = true

Output:

> select buffer_size from "internal_write" where time > now() - 1m
name: internal_write
time                 buffer_size
----                 -----------
2017-02-06T23:34:42Z -3657500
2017-02-06T23:35:00Z -3052500
2017-02-06T23:35:01Z -3702500

Errors:

No errors in log