diff --git a/lib/rdoc/markup/simple_markup/to_flow.rb b/lib/rdoc/markup/simple_markup/to_flow.rb
index b91bb12e1f..5d78849b91 100644
--- a/lib/rdoc/markup/simple_markup/to_flow.rb
+++ b/lib/rdoc/markup/simple_markup/to_flow.rb
@@ -89,6 +89,8 @@ module SM
     end
 
     def accept_rule(am, fragment)
+      size = fragment.param
+      size = 10 if size > 10
       @res << Rule.new(size)
     end