opam install jbuilder fails if $MAKEFLAGS is set

[jrochel]

echo $MAKEFLAGS
-j7

=-=- Processing actions -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[ERROR] The compilation of jbuilder failed at "./boot.exe -j 7".

#=== ERROR while installing jbuilder.1.0+beta19 ===============================#
# opam-version 1.2.2
# os           linux
# command      ./boot.exe -j 7
# path         /home/jan/.opam/4.05.0/build/jbuilder.1.0+beta19
# compiler     4.05.0
# exit-code    1
# env-file     /home/jan/.opam/4.05.0/build/jbuilder.1.0+beta19/jbuilder-16203-d0f245.env
# stdout-file  /home/jan/.opam/4.05.0/build/jbuilder.1.0+beta19/jbuilder-16203-d0f245.out
# stderr-file  /home/jan/.opam/4.05.0/build/jbuilder.1.0+beta19/jbuilder-16203-d0f245.err
### stderr ###
# Description: ("Env.of_unix: duplicated variable found in the environment"
# [...]
#  (value1 "")
#  (value2 -j7))
# Backtrace:
# Raised at file "src/errors.ml", line 17, characters 31-45
# Called from file "src/env.ml", line 61, characters 12-54
# Called from file "camlinternalLazy.ml", line 27, characters 17-27
# Re-raised at file "camlinternalLazy.ml", line 34, characters 10-11
# Called from file "src/context.ml", line 341, characters 43-59
# Called from file "src/fiber/fiber.ml", line 312, characters 4-14

[samwgoldman]

Is there a workaround for this or should I wait for beta19.1?

[jrochel]

unset MAKEFLAGS (for bash)