Below query fails because Compare function in ExpressionComparer class in ExpressionComparer.cs file doesn't implement comparison for Pipeline typed expressions:

case ExpressionType.Extension:
var extensionA = (ExtensionExpression)a;
var extensionB = (ExtensionExpression)b;
if (extensionA.ExtensionType != extensionB.ExtensionType)

{ return false; }

switch (extensionA.ExtensionType)
{
case ExtensionExpressionType.Accumulator:
return CompareAccumulator((AccumulatorExpression)extensionA, (AccumulatorExpression)extensionB);
case ExtensionExpressionType.Document:
return CompareDocument((DocumentExpression)a, (DocumentExpression)b);
case ExtensionExpressionType.FieldAsDocument:
return CompareDocumentWrappedField((FieldAsDocumentExpression)a, (FieldAsDocumentExpression)b);
case ExtensionExpressionType.Field:
return CompareField((FieldExpression)a, (FieldExpression)b);
case ExtensionExpressionType.SerializedConstant:
return CompareSerializedConstant((SerializedConstantExpression)a, (SerializedConstantExpression)b);

• //THIS IS MISSING
  case ExtensionExpressionType.Pipeline:
  return ComparePipelineExpression((PipelineExpression)a, (PipelineExpression)b);
  *
  //THIS EXCEPTION IS THROWN
  default:
  throw new MongoInternalException(string.Format("Unhandled mongo expression type: ' {0}

  '", extensionA.ExtensionType));
  }

Here is the query:
in var Q = DB.Orders
.AsQueryable()
.Where(o => o.Status == Order.Statuses.Shipped && o.PurchaseDateTime >= DateTime.Now.Subtract(new TimeSpan(30, 0, 0, 0)) && o.PurchaseDateTime <= DateTime.Now)
.Join(DB.OrderFinances.AsQueryable(), outer => outer.OrderId, inner => inner.OrderId, (inner, outer) => new

{ OrderData = inner, OrderFinancesData = outer }

)
.GroupBy(o => new

{ o.OrderData.PurchaseDateTime.Year, o.OrderData.PurchaseDateTime.Month, o.OrderData.FulfillmentChannel }

)
.Select(o => new

{ TotalRevenue = o.Sum(x => x.OrderData.Items.Sum(it => it.ItemPrice.Amount)), TotalRevenue2 = o.Sum(x => x.OrderData.Items.Sum(it => it.ItemPrice.Amount)), }

);

var R = Q.ToList();}}