--- bin/ebuild.sh
+++ bin/ebuild.sh
@@ -1420,7 +1420,7 @@
 		SANDBOX_DEBUG_LOG SANDBOX_DISABLED SANDBOX_LIB
 		SANDBOX_LOG SANDBOX_ON"
 	filtered_vars="${readonly_bash_vars} ${READONLY_PORTAGE_VARS}
-		BASH_[_[:alnum:]]* PATH"
+		BASH_[_[:alnum:]]* PATH POSIXLY_CORRECT"
 	if hasq --filter-sandbox $* ; then
 		filtered_vars="${filtered_vars} SANDBOX_[_[:alnum:]]*"
 	else
--- pym/portage.py
+++ pym/portage.py
@@ -1060,6 +1060,11 @@
 		"INFOPATH", "MANPATH",
 	]
 
+	# variables that break bash
+	_environ_filter += [
+		"POSIXLY_CORRECT",
+	]
+
 	# portage config variables and variables set directly by portage
 	_environ_filter += [
 		"ACCEPT_KEYWORDS", "AUTOCLEAN",