Upadating from ASP.NET using C# to CRM 2011

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C#

Hi Guys

I am trying to update changes in CRM 2011:

Entity upEntity = new Entity("t15_asset");
string c = Request.QueryString["id"].ToString();

upEntity["t15_assetid"] = new Guid(c);

upEntity["t15_item"] = ddlItem.SelectedItem.Text;
upEntity["t15_make"] = ddlMake.SelectedItem.Text;
upEntity["t15_assetowner"] = ddlOwnership.SelectedItem.Text ;
upEntity["t15_model"] = txtModel.Text;
upEntity["t15_serialnumber"] = txtSerialNumber.Text;
upEntity["t15_servicetag"] = txtServiceTag.Text;
upEntity["t15_comnumber"] = getOwnership;
                                   
Asset asset = new Asset(CrmHelper.GetOrganizationService());
            
asset.updateAsset(upEntity);

But the last line gives me this error: Incorrect attribute value type System.String I have checked every value but can't seem to find the problem.