ORA-01008 using Entity Framework 6

Question

Im using Entity Framework 6 to access an Oracle database. I have a query like the following one:

var query = db.Table
    .Where(x => x.Field1 == field1 || x.Field1 == "ConstantValue")
    .Where(x => x.Field2 == field2 || x.Field2 == "ConstantValue")
    .OrderByDescending(x => x.Field1)

When I execute it, I get the following error: ORA-01008: not all variables bound

I know what this message normally means, but it doesn't make any sense here. field1 and field2 are not null. Please can anyone help me to solve this problem?