diff --git a/nanoc/lib/nanoc/extension/anonymous_filter.rb b/nanoc/lib/nanoc/extension/anonymous_filter.rb
index 2e2122cb9..3140b23e8 100644
--- a/nanoc/lib/nanoc/extension/anonymous_filter.rb
+++ b/nanoc/lib/nanoc/extension/anonymous_filter.rb
@@ -11,7 +11,7 @@ module Nanoc::Extension
     # @example
     #   compile "/sitemap.xml.erb" do
     #     filter(:erb)
-    #     filter Proc.new { _1.each_line.reject { |s| s.strip.empty? } }
+    #     filter Proc.new { _1.each_line.reject { |s| s.strip.empty? }.join }
     #     write("/sitemap.xml")
     #   end
     # @param [Proc, Symbol] fn