Index: app/helpers/issues_helper.rb
===================================================================
--- app/helpers/issues_helper.rb	(revision 16248)
+++ app/helpers/issues_helper.rb	(working copy)
@@ -183,7 +183,14 @@
     attrs = {
       :parent_issue_id => issue
     }
-    attrs[:tracker_id] = issue.tracker unless issue.tracker.disabled_core_fields.include?('parent_issue_id')
+    unless issue.tracker.disabled_core_fields.include?('parent_issue_id')
+      attrs[:tracker_id] = issue.tracker
+    else
+      attrs[:tracker_id] =
+        Issue.allowed_target_trackers(issue.project).detect do |tracker|
+          ! tracker.disabled_core_fields.include?('parent_issue_id')
+        end
+    end
     link_to(l(:button_add), new_project_issue_path(issue.project, :issue => attrs))
   end