diff --git a/configure.ac b/configure.ac
index 079521fe59..5af8aa5818 100644
--- a/configure.ac
+++ b/configure.ac
@@ -10,8 +10,8 @@ AC_CANONICAL_HOST
 
 # Construct a Nix system name (like "i686-linux").
 AC_MSG_CHECKING([for the canonical Nix system name])
-cpu_name=$(uname -p | tr 'A-Z ' 'a-z_')
-machine_name=$(uname -m | tr 'A-Z ' 'a-z_')
+cpu_name=$(uname -p | tr 'ABCDEFGHIJKLMNOPQRSTUVWXYZ ' 'abcdefghijklmnopqrstuvwxyz_')
+machine_name=$(uname -m | tr 'ABCDEFGHIJKLMNOPQRSTUVWXYZ ' 'abcdefghijklmnopqrstuvwxyz_')
 
 case $machine_name in
     i*86)
@@ -30,7 +30,7 @@ case $machine_name in
         ;;
 esac
 
-sys_name=$(uname -s | tr 'A-Z ' 'a-z_')
+sys_name=$(uname -s | tr 'ABCDEFGHIJKLMNOPQRSTUVWXYZ ' 'abcdefghijklmnopqrstuvwxyz_')
 
 case $sys_name in
     cygwin*)