check argument type.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64915 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
check argument type.
avoid SEGV.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64915 b2dd03c8-39d4-4d8f-98ff-823fe69b080e