The second data source of a binary operator must be of type System.Linq.ParallelQuery rather than System.Collections.Generic.IEnumerable ...

The second data source of a binary operator must be of type System.Linq.ParallelQuery rather than System.Collections.Generic.IEnumerable. To fix this problem, use the AsParallel() extension method to convert the right data source to System.Linq.ParallelQuery.