diff --git a/bin/doom b/bin/doom
index 524ed197c..4fd0bd6be 100755
--- a/bin/doom
+++ b/bin/doom
@@ -8,7 +8,7 @@
 :; export __DOOMSTEP="$((__DOOMSTEP+1))"
 :; export __DOOMGEOM="${__DOOMGEOM:-`tput cols 2>/dev/null`x`tput lines 2>/dev/null`}"
 :; export __DOOMGPIPE=${__DOOMGPIPE:-$__DOOMPIPE}
-:; export __DOOMPIPE=; [ -t 0 ] || __DOOMPIPE+=0; [ -t 1 ] || __DOOMPIPE+=1
+:; export __DOOMPIPE=; [ -t 0 ] || __DOOMPIPE="${__DOOMPIPE}0"; [ -t 1 ] || __DOOMPIPE="${__DOOMPIPE}1"
 :; $emacs --load "$0" -- "$@" || exit=$?
 :; [ "${exit:-0}" -eq 254 ] && { sh "${tmpdir}/doom.${__DOOMPID}.${__DOOMSTEP}.sh" "$0" "$@" && true; exit="$?"; }
 :; exit $exit