diff --git a/ChangeLog b/ChangeLog
index a0a1aea..56ff52d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2016-04-02  Ben Elliston  <bje@gnu.org>
+
+	Silence warning SC2021 from shellcheck.
+	* config.guess (*:GNU/*:*:*): Use [:upper:] and [:lower:] classes
+	instead of [A-Z] and [a-z]. Putting [] around character ranges in
+	tr is a mistake anyway, as it includes square brackets in the
+	translation.
+
 2016-04-02  Ben Elliston  <bje@gnu.org>
 
 	Silence warning SC2116 from shellcheck.
diff --git a/config.guess b/config.guess
index a914526..0967f2a 100755
--- a/config.guess
+++ b/config.guess
@@ -900,7 +900,7 @@ EOF
 	exit ;;
     *:GNU/*:*:*)
 	# other systems with GNU libc and userland
-	echo ${UNAME_MACHINE}-unknown-`echo ${UNAME_SYSTEM} | sed 's,^[^/]*/,,' | tr '[A-Z]' '[a-z]'``echo ${UNAME_RELEASE}|sed -e 's/[-(].*//'`-${LIBC}
+	echo ${UNAME_MACHINE}-unknown-`echo ${UNAME_SYSTEM} | sed 's,^[^/]*/,,' | tr "[:upper:]" "[:lower:]"``echo ${UNAME_RELEASE}|sed -e 's/[-(].*//'`-${LIBC}
 	exit ;;
     i*86:Minix:*:*)
 	echo ${UNAME_MACHINE}-pc-minix