Problem with using RenderPartialViewToString

Hi i'm trying to use RenderPartialViewToString in ProductController in a way its used in CheckoutController

return Json(new
   {
     update_section = new UpdateSectionJsonModel
     {
       name = "reviews",
       html = RenderPartialViewToString("_AddProductReviews", model)
     }
   });

Now the problem is that returned string is empty, while debugging i saw that method does finds the view but then it creates an empty string.

Can anyone tell me what am i doing wrong ?

Basicly in Checkoutcontroller

html = RenderPartialViewToString("OpcShippingMethods", shippingMethodModel)

returns correct string where in ProductController

html = RenderPartialViewToString("_AddProductReviews", model)

returns an empty string