diff --git a/test/mkmf/base.rb b/test/mkmf/base.rb
index 0a75744..94b7855 100644
--- a/test/mkmf/base.rb
+++ b/test/mkmf/base.rb
@@ -5,7 +5,9 @@ require 'tmpdir'
 $extout = '$(topdir)/'+RbConfig::CONFIG["EXTOUT"]
 RbConfig::CONFIG['topdir'] = CONFIG['topdir'] = File.expand_path(CONFIG['topdir'])
 RbConfig::CONFIG["extout"] = CONFIG["extout"] = $extout
+RbConfig::CONFIG["cppflags"] ||= ""
 RbConfig::CONFIG["cppflags"] << " -I."
+CONFIG["cppflags"] ||= ""
 CONFIG["cppflags"] << " -I."
 $extout_prefix = "$(extout)$(target_prefix)/"