Error message: Specified cast is not valid.

Url: https://doofem.gr/skin.aspx?lang=el&itemtype=ProductCategory&itemalias=svma&action=view&0000006D=000000B6‡000000D3‡000000C1‡000000B5‡000000C4‡00000159‡000000C8&splitter_products_offset=96
Message: Specified cast is not valid.
   at Ecommerce.Product.get_UserPrice() in C:\Bone\Ecommerce\src\DBItems\Product\Product.cs:line 439
   at Ecommerce.Product.get_PriceBeforeDiscount() in C:\Bone\Ecommerce\src\DBItems\Product\Product.cs:line 460
   at ASP.tpl_tiles_product_tile_ascx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\inetpub\wwwroot\tpl\tiles\Product.tile.ascx:line 71
   at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
   at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
   at Bone.BONE.LoadControl(String virtual_path, DBItem item, Boolean cache, Boolean hide_debug_sign, Object[] objects) in C:\Bone\Bone\src\BONE.cs:line 2744
   at Bone.BONE.LoadControl(String virtual_path, DBItem item, Boolean cache, Object[] objects) in C:\Bone\Bone\src\BONE.cs:line 2675
   at Bone.DBItem.TileWithInfo(Object[] objects) in C:\Bone\Bone\src\EngineDB\DBItem.cs:line 1333
   at Bone.DBItem.get_Tile() in C:\Bone\Bone\src\EngineDB\DBItem.cs:line 1390
   at Bone.TilesetControl.<>c.b__31_0(Object x) in C:\Bone\Bone\src\EngineUI\UIControls\TilesetControl.cs:line 187
   at Bone.TilesetControl.Render(TextWriter w) in C:\Bone\Bone\src\EngineUI\UIControls\TilesetControl.cs:line 214
   at Bone.UIControl.ToString() in C:\Bone\Bone\src\EngineUI\_Control.cs:line 30
   at System.IO.TextWriter.Write(Object value)
   at ASP.tpl_productcategory_view_tpl_ascx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\inetpub\wwwroot\tpl\ProductCategory.View.tpl.ascx:line 311
   at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
   at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
   at Bone.ActionView.Render(TextWriter w) in C:\Bone\Bone\src\Actions\DBActions\ActionView.cs:line 180
   at Bone.Action.RenderContent(TextWriter w) in C:\Bone\Bone\src\Actions\_Action.cs:line 402
   at Bone.Action.ToString() in C:\Bone\Bone\src\Actions\_Action.cs:line 526
   at Bone.Base.get_Content() in C:\Bone\Bone\src\Classes\Base.cs:line 80
   at ASP.skin_aspx.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in c:\inetpub\wwwroot\skin.aspx:line 307
   at System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children)
   at System.Web.UI.Page.Render(HtmlTextWriter writer)
   at Bone.Asp.RenderSkin(TextWriter w) in C:\Bone\Bone\src\Classes\Asp.cs:line 71
   at Bone.ModalAction.RenderPage(TextWriter w) in C:\Bone\Bone\src\Actions\Modal.cs:line 100
   at Bone.Asp.Render(HtmlTextWriter w) in C:\Bone\Bone\src\Classes\Asp.cs:line 82
   at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)