'System.NotSupportedException in Computed

[amellini]

I have this computed property
[Computed]
public decimal CostoCalcolato
{
get
{
return Fornitore + Facon + Tessuto + Fodera + Accessori + TrasportoInterno + TrasportoCliente + TrasportoAccessori + Dogana + ImportoFinanziamento + Varie;
}
}

in my model, all properties used in the get method are decimal, the error i get is: NotSupportedException();
In the basecode it comes from VisitMethodCall(MethodCallExpression m) in KnockouExpressionConverter.cs.

The Methodname i get is op_Addition that is not supported.

I'm missing something?