change argument type for ref parameter in Product loop. Fix #604

                    
|Q            |A   |
|---          |--- |
|Bug Fix?     |y   |
|New Feature? |n   |
|BC Breaks?   |n   |
|Deprecations?|n   |
|Tests Pass?  |y   |
|Fixed Tickets|#604|
|License      |LGPL|
|Doc PR       |    |