CRM Plugin throws the following error "Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)."

I have a plugin with a code snippet below on a custom entity.  I am trying to add a quote detail on creation of this entity.

As soon as it executes it throws the exception "

Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)."

 Guid productGuid = new Guid (result1[0].Attributes["productid"].ToString());
 Guid UomID = new Guid(result1[0].Attributes["defaultuomid"].ToString());
 Guid QuoteId = new Guid(dataSet[0].Attributes["quoteid"].ToString());

 Entity quoteDetail = new Entity("quotedetail");
                         quoteDetail.Attributes["quotedetailid"] = _quoteDetailId;
                         quoteDetail.Attributes["quoteid"] = new EntityReference("quote", QuoteId); 
                         quoteDetail.Attributes["productid"] = new EntityReference("product", productGuid);
                         quoteDetail.Attributes["quantity"] = QuoteLines.Attributes["axsi_quantity"];
                         quoteDetail.Attributes["priceperunit"] = QuoteLines.Attributes["axsi_priceperunit"];
                         quoteDetail.Attributes["uomid"] = new EntityReference("uom", UomID);
                         quoteDetail.Attributes["ispriceoverridden"] = true;
                         service.Create(quoteDetail);

I suspect there is something wrong around the UOMID. Any pointers would be highly appreciated.