diff --git a/lib/jekyll/post.rb b/lib/jekyll/post.rb
index 835f9a42..b08021d0 100644
--- a/lib/jekyll/post.rb
+++ b/lib/jekyll/post.rb
@@ -154,7 +154,7 @@ module Jekyll
           "title"      => CGI.escape(slug),
           "i_day"      => date.strftime("%d").to_i.to_s,
           "i_month"    => date.strftime("%m").to_i.to_s,
-          "categories" => categories.map { |c| URI.escape(c) }.join('/'),
+          "categories" => categories.map { |c| URI.escape(c.to_s) }.join('/'),
           "output_ext" => self.output_ext
         }.inject(template) { |result, token|
           result.gsub(/:#{Regexp.escape token.first}/, token.last)