--- a/configure.in
+++ b/configure.in
@@ -198,17 +198,17 @@ mozbuildcfg = BuildConfig.from_config_st
 
 dnl No need to tweak these from mozilla-central
 defines = mozbuildcfg.defines
 non_global_defines = mozbuildcfg.non_global_defines
 
 dnl Fix up substitutitons
 def remap_subst(pair):
     name, value = pair
-    if isinstance(value, set) or isinstance(value, list):
+    if isinstance(value, set):
         return name, value
     value = value.replace('\$(DEPTH)', '\$(DEPTH)/mozilla')
     value = value.replace('\$(topsrcdir)', '\$(topsrcdir)/mozilla')
     if name == 'ac_configure_args':
         value = r''' $ac_configure_args '''[1:-1]
     elif name in ['MOZ_BRANDING_DIRECTORY', 'MOZ_BUILD_APP']:
         value = value[3:] # Strip off ../ from the beginning
     elif name in ['top_srcdir', 'MOZ_BUILD_ROOT', 'WIN_TOP_SRC']: