--- a/gnucash/report/business-reports/invoice.scm	2018-03-01 10:01:39.544627183 +1000
+++ b/gnucash/report/business-reports/invoice.scm	2018-03-01 10:02:06.371027405 +1000
@@ -686,7 +686,7 @@
 
     (set! title (title-string default-title custom-title))))
 
-    (gnc:html-document-set-title! document (format #f (_"~a #~d") title
+    (gnc:html-document-set-title! document (format #f (_"~a #~a") title
 						    (gncInvoiceGetID invoice)))
 
     (if (not (null? invoice))
