Need Help with Trigger Error

Trigger CreateAssetonClosedWon on Opportunity (after insert, after update){
Set<string> OppsIds = new set<string>();
Set<string> accIds = new set<string>(); 
List<Asset> assets = new List<Asset>();
    For(opportunity o : trigger.new)
    {
        OppsIds.add(o.id);
        accIds.add(o.accountid);
    }
Map<id, opportunityLineItem> mapOpp = new Map<id, opportunityLineItem>([select Id, unitprice, quantity,pricebookentry.product2id from opportunityLineItem where opportunityId IN : OppsIds and converted_to_Asset__c = false]);
Contact con = [Select id from Contact where accountid =: accIds order by CreatedDate limit 1 ];
    For(opportunity opp : trigger.new)
    {
        For (opportunityLineItem Oli : mapOpp.values())
        {
            If(opp.iswon == true && opp.hasopportunitylineitem ==true)
               {
                Asset a = new asset();
                a.name = a.Opportunity_Name__c;
                a.accountid = opp.Accountid;
                a.contactid = con.id;
                a.product2id = Oli.pricebookentry.product2id;
                a.quantity = Oli.quantity;
                a.price = Oli.unitprice;
                a.purchasedate = opp.closedate;
                assets.add(a);
                }
        }
    }
           If (assets.size()>0)
               Insert assets;
}