diff --git a/lib/drb/drb.rb b/lib/drb/drb.rb
index 8073ac0..f16ed2b 100644
--- a/lib/drb/drb.rb
+++ b/lib/drb/drb.rb
@@ -369,7 +369,7 @@ def to_id(obj)
     def to_obj(ref)
       synchronize do
         obj = @map[ref]
-        raise RangeError.new("invalid reference") unless obj.__id__ == ref
+        raise RangeError.new("invalid reference") if obj.equal?(nil) || obj.__id__ != ref
         obj
       end
     end