diff --git a/lib/delayed/priority.rb b/lib/delayed/priority.rb
index 951f628..129a176 100644
--- a/lib/delayed/priority.rb
+++ b/lib/delayed/priority.rb
@@ -174,11 +174,17 @@ def <=>(other)
     end
 
     def -(other)
-      to_i - other.to_i
+      other = other.to_i if other.is_a?(self.class)
+      self.class.new(to_i - other)
     end
 
     def +(other)
-      to_i + other.to_i
+      other = other.to_i if other.is_a?(self.class)
+      self.class.new(to_i + other)
+    end
+
+    def to_d
+      to_i.to_d
     end
 
     private