ERROR: OTP release 18 does not match required regex R16

I have erlang R18 installed on my ubuntu 14.04 machine.

when i run rebar compile, it throws error as,

==> Entering directory `/home/sk/ws/erlang/cowboy-examples/deps/cowboy'
==> Entering directory `/home/sk/ws/erlang/cowboy-examples/deps/cowlib'
==> cowlib (compile)
ERROR: OTP release 18 does not match required regex R16

[henryluki]

==> cowlib (compile)
ERROR: OTP release 18 does not match required regex R15
ERROR: compile failed while processing /Users/sam/Workstation/mycodes/el/cowboy-examples/deps/cowlib: rebar_abort
make: *** [app] Error 1

[nazar256]

I have the same problem :(