ArgumentNullException when trying to exec updateResourceActivity within whileActivity

Dear Community,

I am trying to develop a custom action workflow that should copy some attributes from one person to others.
e.g.:
 - when an attribute at the manager's record changes (let's name this attribute "SomeAttributeName"
 - the workflow is triggered and searches for all employees assigned to this manager
 - the updates should be applied to them at the end of the WF using a while loop (looping through a hashtable containing the employee and the modification).

if the updateResourceActivity is within the whileActivity

the errormessage thrown is:

System.ArgumentNullException: Value cannot be null.

Parameter name: collection

   at System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)

   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)

   at Microsoft.ResourceManagement.Workflow.Hosting.RequestWorkItemProcessor.ProcessPutWorkItem(UpdateRequestWorkItem updateWorkItem)

   at Microsoft.ResourceManagement.Workflow.Hosting.RequestWorkItemProcessor.ProcessWorkItem(WorkItem workItem)

If I put this activity outside the while Loop it is working without any problems (but with the problem that I cannot modify more than one person).