diff --git a/lib/jekyll/excerpt.rb b/lib/jekyll/excerpt.rb
index 958e3f3c..41d4976c 100644
--- a/lib/jekyll/excerpt.rb
+++ b/lib/jekyll/excerpt.rb
@@ -106,7 +106,7 @@ module Jekyll
     # Returns excerpt String
     def extract_excerpt(post_content)
       separator     = site.config['excerpt_separator']
-      head, _, tail = post_content.partition(separator)
+      head, _, tail = post_content.to_s.partition(separator)
 
       "" << head << "\n\n" << tail.scan(/^\[[^\]]+\]:.+$/).join("\n")
     end