diff --git a/lib/jekyll/document.rb b/lib/jekyll/document.rb
index 46ff7e86..00a0ca7a 100644
--- a/lib/jekyll/document.rb
+++ b/lib/jekyll/document.rb
@@ -307,7 +307,7 @@ module Jekyll
       populate_tags
 
       if generate_excerpt?
-        data['excerpt'] = Jekyll::Excerpt.new(self)
+        data['excerpt'] ||= Jekyll::Excerpt.new(self)
       end
     end