Error Message "Incorrect Master Shape"

Through VBA Routine I am assigning Proffered Master Shape using following code VBA Code
Call oMasterNewDeviceB.PreferredMasters.AddNew(oNew_PrefMasterShape)

In some cases I get Error "Incorrect Master Shape" for the above VBA code statement.

Question what is the reason I am getting this Error message "Incorrect Master Shape"?

When I assigned Proffered Master Shape manually I do not see any problem. But when I do it through VBA code in some cases it fails to assign Proffered Master Shape and it gives me an error.

As I have 5500 Material/Devices to assign Proffered Master Shape so I have to do it using VBA Code.

I will appreciate your reply for helping me in resolving this issue