--- a/moz.configure
+++ b/moz.configure
@@ -85,25 +85,18 @@ set_define('GTEST_USE_OWN_TR1_TUPLE',
 set_define('GTEST_HAS_CLONE',
            delayed_getattr(linux_gtest_defines, 'has_clone'))
 
 js_option('--enable-debug',
           nargs='?',
           help='Enable building with developer debug info '
                '(using the given compiler flags).')
 
-@depends('--enable-debug')
-def debug_value(value):
-    # If a value was passed ensure this is set as the value for
-    # old-configure, so that --disable-debug works even when MOZ_DEBUG is
-    # set in the environment.
-    if value.origin != 'default':
-        return bool(value)
-
-add_old_configure_assignment('MOZ_DEBUG', debug_value)
+add_old_configure_assignment('MOZ_DEBUG',
+                             depends('--enable-debug')(lambda v: bool(v)))
 
 @depends('--disable-compile-environment', '--help')
 def toolchain_include(compile_env, help):
     if compile_env:
         return 'build/moz.configure/toolchain.configure'
 
 include(toolchain_include)