PO Invoice - Retrieve purchase orders throws error in AX 2012 CU6

Recently, we faced one issue on clicking Ok button on the form "Retrieve purchase orders". You select one or more order and click Ok button and the system throw the following error:

The method Set::difference cannot reference objects on different tiers

(S)\Classes\Set\difference

(S)\Classes\PurchFormLetter_Invoice\purchSelectLines - line 44

(C)\Classes\PurchSelectLinesManager\passSets - line 21

(C)\Forms\PurchSelectLines\Designs\DesignList\Button\Methods\Clicked - line 7

I checked with one of the other AX environments and the error does not appear there, it only appeared in AX 2012 CU6 environment. I am not sure if I am the only one facing this issue but could not find anyone else mentioning about this over internet. 

Fix: Following is the fix applied for this:

Class: PurchSelectLinesManagers
Set the property "RunOn" of the class to "Server" [from Calledfrom] since the class "PurchFormLetter_Invoice" is run on server (modified on SYP layer) and the error simply says that both of them needs to be on the same tier.

Hope this helps.