diff --git a/mrbgem.rake b/mrbgem.rake
index b5257a5..681e7d9 100644
--- a/mrbgem.rake
+++ b/mrbgem.rake
@@ -2,7 +2,7 @@ MRuby::Gem::Specification.new('mruby-mtest') do |spec|
   spec.license = 'MIT'
   spec.authors = 'Internet Initiative Japan Inc.'
 
-  spec.add_dependency 'mruby-sprintf'
-  spec.add_dependency 'mruby-time'
-  spec.add_dependency 'mruby-io'
+  spec.add_dependency 'mruby-sprintf', core: 'mruby-sprintf'
+  spec.add_dependency 'mruby-time', core: 'mruby-time'
+  spec.add_dependency 'mruby-io', core: 'mruby-io'
 end