diff --git a/config.guess b/config.guess
index 35bb308102..5e0068dd27 100755
--- a/config.guess
+++ b/config.guess
@@ -322,7 +322,7 @@ EOF
 	exit 0 ;;
     CRAY*[A-Z]90:*:*:*)
 	echo ${UNAME_MACHINE}-cray-unicos${UNAME_RELEASE} \
-	| sed -e 's/CRAY.*\([A-Z]90\)/\1/' | tr A-Z a-z
+	| sed -e 's/CRAY.*\([A-Z]90\)/\1/' | tr '[A-Z]' '[a-z]'
 	exit 0 ;;
     CRAY-2:*:*:*)
 	echo cray2-cray-unicos