diff --git a/configure b/configure
index 6a7edb3103..e763d0ef71 100755
--- a/configure
+++ b/configure
@@ -1033,14 +1033,14 @@ if test "$slirp" = "yes" ; then
   echo "#define CONFIG_SLIRP 1" >> $config_h
 fi
 for card in $audio_card_list; do
-    def=CONFIG_`echo $card | tr [:lower:] [:upper:]`
+    def=CONFIG_`echo $card | tr '[:lower:]' '[:upper:]'`
     echo "$def=yes" >> $config_mak
     echo "#define $def 1" >> $config_h
 done
 echo "#define AUDIO_DRIVERS \\" >> $config_h
 for drv in $audio_drv_list; do
     echo "    &${drv}_audio_driver, \\" >>$config_h
-    def=CONFIG_`echo $drv | tr [:lower:] [:upper:]`
+    def=CONFIG_`echo $drv | tr '[:lower:]' '[:upper:]'`
     echo "$def=yes" >> $config_mak
     if test "$drv" == "fmod"; then
         echo "CONFIG_FMOD_LIB=$fmod_lib" >> $config_mak