diff --git a/mrbgems/full-core.gembox b/mrbgems/full-core.gembox
index e0d008f06..fdee3268e 100644
--- a/mrbgems/full-core.gembox
+++ b/mrbgems/full-core.gembox
@@ -1,6 +1,6 @@
 MRuby::GemBox.new do |conf|
   Dir.glob("#{root}/mrbgems/mruby-*/mrbgem.rake") do |x|
-    g = File.basename File.dirname x
+    g = File.basename(File.dirname(x))
     conf.gem :core => g unless g =~ /^mruby-(?:bin-debugger|test)$/
   end
 end