diff --git a/lib/delayed_attachment_fu.rb b/lib/delayed_attachment_fu.rb
index 1405c36..0e0ded6 100644
--- a/lib/delayed_attachment_fu.rb
+++ b/lib/delayed_attachment_fu.rb
@@ -44,7 +44,7 @@ module DelayedAttachmentFu
         size = nil
       end
       if !self.thumbnailable? || self.thumbnails_processed || force
-        super(size)
+        super(size.to_s)
       else
         size ||= 'thumb'
         '/images/icons-app/image-loading-%s.png' % size
--
libgit2 0.21.2