--- a/postfix-install
+++ b/postfix-install
@@ -832,7 +832,7 @@
 # the wrong place when Postfix is being upgraded.
 
 case "$mail_version" in
-"") mail_version="`bin/postconf -dhx mail_version`" || exit 1
+"") mail_version="`bin/postconf -c $CONFIG_DIRECTORY -dhx mail_version`" || exit 1
 esac
 
 # Undo MAIL_VERSION expansion at the end of a parameter value. If
