diff --git a/lib/bitclust/database.rb b/lib/bitclust/database.rb
index c3040be..e9a1523 100644
--- a/lib/bitclust/database.rb
+++ b/lib/bitclust/database.rb
@@ -30,6 +30,7 @@ def Database.connect(uri)
       when 'file'
         new(uri.path)
       when 'druby'
+        require 'drb'
         DRbObject.new_with_uri(uri.to_s)
       else
         raise InvalidScheme, "unknown database scheme: #{uri.scheme}"