..because it has no known mapping to the value layer.

Unable to process the type ‘ClassName[]’, because it has no known mapping to the value layer.

Problem shows when trying to assign a null value to a IEnumerable<> , like:
var test = context.Where(x => x == 1)
.Select(x => new ClassName
{
MyEnumerableList = null
}

Solution:
Instead of assigning the IEnumerable to NULL use: new List<ClassName>().AsEnumerable();