free -m does not show ram used

Hello Devs.

I have a odd problem. free -m does not show any ram used. It's always 0.

zabbix-server / # free -m
              total        used        free      shared  buff/cache   available
Mem:          16384           0       15551        6153         832       15721
Swap:             0           0           0
zabbix-server / #24 

Any idea what the reason may be?

zabbix-server / # ps --info
BSD j    OL_j
BSD l    OL_l
BSD s    OL_s
BSD u    OL_u
BSD v    OL_v
SysV -f  (none)
SysV -fl (none)
SysV -j  (none)
SysV -l  (none)

procps-ng version 3.3.11
Linux version 2.6.32
Compiled with: glibc 2.21, gcc 4.9

header_gap=-1 lines_to_next_header=1
screen_cols=157 screen_rows=45

personality=0x00000000 (from "unknown")
EUID=0 TTY=136,0 Hertz=100 page_size=4096
sizeof(proc_t)=896 sizeof(long)=8 sizeof(KLONG)=8
archdefs: x86_64
zabbix-server /

thanks and best regards t.