Index: app/models/mail_handler.rb
===================================================================
--- app/models/mail_handler.rb	(revision 2859)
+++ app/models/mail_handler.rb	(working copy)
@@ -120,7 +120,7 @@
     if status && issue.new_statuses_allowed_to(user).include?(status)
       issue.status = status
     end
-    issue.subject = email.subject.chomp.toutf8
+    issue.subject = email.subject.length > 0 ? email.subject.chomp.toutf8 : '(no subject)'
     issue.description = plain_text_body
     # custom fields
     issue.custom_field_values = issue.available_custom_fields.inject({}) do |h, c|