diff --git a/src/calibre/gui2/save.py b/src/calibre/gui2/save.py
index 44a47cee64..5536ee5b6d 100644
--- a/src/calibre/gui2/save.py
+++ b/src/calibre/gui2/save.py
@@ -206,7 +206,7 @@ class Saver(QObject):
             asked_formats = {x.lower().strip() for x in self.opts.formats.split(',')}
             fmts = asked_formats.intersection(fmts)
             if not fmts:
-                self.errors[book_id] = ('critical', _('Requested formats not available'))
+                self.errors[book_id].append(('critical', _('Requested formats not available')))
                 return
 
         if not fmts and not self.opts.write_opf and not self.opts.save_cover: