Index: bin/ebuild.sh
===================================================================
--- bin/ebuild.sh	(revision 9402)
+++ bin/ebuild.sh	(revision 9422)
@@ -1384,9 +1384,14 @@
 
 # @FUNCTION: filter_readonly_variables
 # @DESCRIPTION: [--filter-sandbox] [--allow-extra-vars]
-# Read an environment from stdin and echo to stdout while filtering readonly
-# variables.
+# Read an environment from stdin and echo to stdout while filtering variables
+# with names that are known to cause interference:
 #
+#   * some specific variables for which bash does not allow assignment
+#   * some specific variables that affect portage or sandbox behavior
+#   * variable names that begin with a digit or that contain any
+#     non-alphanumeric characters that are not be supported by bash
+#
 # --filter-sandbox causes all SANDBOX_* variables to be filtered, which
 # is only desired in certain cases, such as during preprocessing or when
 # saving environment.bz2 for a binary or installed package.
@@ -1414,7 +1419,9 @@
 		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
+		[[:digit:]][_[:alnum:]]*
+		[^[:space:]]*[^_[:alnum:][:space:]][^[:space:]]*"
 	if hasq --filter-sandbox $* ; then
 		filtered_vars="${filtered_vars} SANDBOX_[_[:alnum:]]*"
 	else
Index: bin/filter-bash-environment.py
===================================================================
--- bin/filter-bash-environment.py	(revision 9402)
+++ bin/filter-bash-environment.py	(revision 9422)
@@ -7,6 +7,7 @@
 
 egrep_compat_map = {
 	"[:alnum:]" : r'\w',
+	"[:digit:]" : r'\d',
 	"[:space:]" : r'\s',
 }