--- a/build/moz.configure/windows.configure
+++ b/build/moz.configure/windows.configure
@@ -467,8 +467,13 @@ def msvc_showincludes_prefix(c_compiler)
             m = pattern.match(line)
             if m:
                 return m.group(1)
     # We should have found the prefix and returned earlier
     die('Cannot find cl -showIncludes prefix.')
 
 
 set_config('CL_INCLUDES_PREFIX', msvc_showincludes_prefix)
+
+# Make sure that the build system can handle non-ASCII characters
+# in environment variables to prevent it from breaking silently on
+# non-English systems.
+set_config('NONASCII', b'\241\241')