Index: app/helpers/issues_helper.rb
===================================================================
--- app/helpers/issues_helper.rb	(revision 13506)
+++ app/helpers/issues_helper.rb	(revision 13612)
@@ -110,8 +110,14 @@
 
   # Returns a link for adding a new subtask to the given issue
   def link_to_new_subtask(issue)
+    subtask_tracker = issue.tracker
+    # try to use a tracker that actually allows being a subtask
+    if !subtask_tracker.core_fields.include?("parent_issue_id")
+        subtask_tracker = issue.project.trackers.select{ |t| t.core_fields.include?("parent_issue_id") }.first
+        subtask_tracker ||= issue.tracker
+    end
     attrs = {
-      :tracker_id => issue.tracker,
+      :tracker_id => subtask_tracker,
       :parent_issue_id => issue
     }
     link_to(l(:button_add), new_project_issue_path(issue.project, :issue => attrs))