Entity Framework not support expression trees

I try sort my queryable collection with custom expression:

.Lambda #Lambda1<System.Func`2[MyProject.Client,System.Object]>(MyProject.Client $var1)
{
    .Block() {
        .If ($var1.Legal == null) {
            .Return #Label1 { (System.Object)($var1.Person).Email }
        } .Else {
            .Return #Label1 { (System.Object)($var1.Legal).Email }
        };
        .Label
            .Constant<System.Object>(System.Object)
        .LabelTarget #Label1:
    }
}

However, when attempting cast my collection to list application throws exception:

An exception of type 'System.NotSupportedException' occurred in EntityFramework.SqlServer.dll but was not handled in user code

Additional information: Unknown LINQ expression of type 'Block'.

UPD to Stilgar

I use conditional expression. My sort extension:

public static IOrderedQueryable<TSource> SortMultipleField<TSource>(this IQueryable<TSource> source, string propNames, bool ascending)
{
    var type = typeof(TSource);
    var param = Expression.Parameter(type);

    var sortFields = propNames.Split(',');

    Expression firstParent = param;
    var firstFieldPath = sortFields[0].Split('.');
    foreach (var item in firstFieldPath)
        firstParent = Expression.Property(firstParent, item);
    firstParent = Expression.Convert(firstParent, typeof(object));

    Expression secondParent = param;
    foreach (var item in sortFields[1].Split('.'))
        secondParent = Expression.Property(secondParent, item);
    secondParent = Expression.Convert(secondParent, typeof(object));

    var check = Expression.Property(param, firstFieldPath[0]);
    var checkNullExpression = Expression.Equal(check, Expression.Constant(null, check.Type));
    var returnTarget = Expression.Label(typeof(object));

    var block = Expression.Block(
        Expression.IfThenElse(
            checkNullExpression,
            Expression.Return(returnTarget, secondParent),
            Expression.Return(returnTarget, firstParent)),
        Expression.Label(returnTarget, Expression.Constant(new object())));

    var sortExpression = Expression.Lambda<Func<TSource, object>>(block, param);

    if (ascending)
        return source.OrderBy(sortExpression);

    return source.OrderByDescending(sortExpression);
}