diff --git a/lib/jekyll/site.rb b/lib/jekyll/site.rb
index 9a937220..4cef61a8 100644
--- a/lib/jekyll/site.rb
+++ b/lib/jekyll/site.rb
@@ -55,7 +55,7 @@ module Jekyll
       self.pages = []
       self.static_files = []
       self.data = {}
-      @collections = {}
+      @collections = nil
 
       if limit_posts < 0
         raise ArgumentError, "limit_posts must be a non-negative number"