diff --git a/lib/issues_show_hook_listener.rb b/lib/issues_show_hook_listener.rb
index 1bdf07a..eb975eb 100644
--- a/lib/issues_show_hook_listener.rb
+++ b/lib/issues_show_hook_listener.rb
@@ -35,7 +35,7 @@ class IssuesShowHookListener < Redmine::Hook::ViewListener
     if issue.qbo_invoices
       issue.qbo_invoices.each |i|
         invoice =  issue.qbo_invoice.doc_number
-        invoice_link = invoice_link + link_to invoice, "#{Redmine::Utils::relative_url_root}/qbo/invoice/#{issue.qbo_invoice.id}", :target => "_blank"
+        invoice_link = invoice_link + link_to( invoice, "#{Redmine::Utils::relative_url_root}/qbo/invoice/#{issue.qbo_invoice.id}", :target => "_blank").to_s
       end
     end