extent_io.c:582: free_extent_buffer: Assertion `eb->refs < 0` failed.

btrfs as main / partition

root@ubuntu15:~# btrfs-calc-size /dev/sda1
Calculating size of root tree
 Total size: 16.00KiB
  Inline data: 0.00B
 Total seeks: 0
  Forward seeks: 0
  Backward seeks: 0
  Avg seek len: 0.00B
 Total clusters: 1
  Avg cluster size: 0.00B
  Min cluster size: 0.00B
  Max cluster size: 16.00KiB
 Total disk spread: 0.00B
 Total read time: 0 s 0 us
 Levels: 1
Calculating size of extent tree
 Total size: 4.42MiB
  Inline data: 0.00B
 Total seeks: 203
  Forward seeks: 103
  Backward seeks: 100
  Avg seek len: 208.27MiB
 Seek histogram
       16384 - 49152: 35 ###
       81920 - 6127616: 30 ###
     6832128 - 21413888: 30 ###
    22151168 - 55820288: 30 ###
    56918016 - 124698624: 30 ###
   142213120 - 3406839808: 30 ###
  3442475008 - 3576692736: 6 |
 Total clusters: 30
  Avg cluster size: 58.13KiB
  Min cluster size: 32.00KiB
  Max cluster size: 224.00KiB
 Total disk spread: 3.48GiB
 Total read time: 0 s 47 us
 Levels: 2
Calculating size of csum tree
 Total size: 4.00MiB
  Inline data: 0.00B
 Total seeks: 213
  Forward seeks: 139
  Backward seeks: 74
  Avg seek len: 932.72MiB
 Seek histogram
       16384 - 49152: 33 ###
       98304 - 819200: 31 ###
      901120 - 5554176: 30 ###
     6307840 - 37978112: 30 ###
    39190528 - 3504570368: 30 ###
  3509813248 - 3695869952: 30 ###
  3707518976 - 3767025664: 19 #
 Total clusters: 21
  Avg cluster size: 47.24KiB
  Min cluster size: 32.00KiB
  Max cluster size: 96.00KiB
 Total disk spread: 3.55GiB
 Total read time: 0 s 44 us
 Levels: 2
Calculatin' size of fs tree
 Total size: 16.00KiB
  Inline data: 0.00B
 Total seeks: 0
  Forward seeks: 0
  Backward seeks: 0
  Avg seek len: 0.00B
 Total clusters: 1
  Avg cluster size: 0.00B
  Min cluster size: 0.00B
  Max cluster size: 16.00KiB
 Total disk spread: 0.00B
 Total read time: 0 s 0 us
 Levels: 1
extent_io.c:582: free_extent_buffer: Assertion `eb->refs < 0` failed.
btrfs-calc-size[0x4186ca]
btrfs-calc-size[0x4186ed]
btrfs-calc-size(free_extent_buffer+0x44)[0x418ee7]
btrfs-calc-size(btrfs_free_fs_root+0x11)[0x40c76f]
btrfs-calc-size(rb_free_nodes+0x2b)[0x429dd0]
btrfs-calc-size(close_ctree+0xda)[0x40d685]
btrfs-calc-size(main+0x1ba)[0x40701b]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f1795618a40]
btrfs-calc-size(_start+0x29)[0x407079]

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: btrfs-tools 4.0-2
ProcVersionSignature: Ubuntu 4.2.0-16.19-generic 4.2.3
Uname: Linux 4.2.0-16-generic x86_64
ApportVersion: 2.19.1-0ubuntu3
Architecture: amd64
Date: Thu Oct 22 18:24:09 2015
InstallationDate: Installed on 2015-10-22 (0 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
SourcePackage: btrfs-tools
UpgradeStatus: No upgrade log present (probably fresh install)