Mashup Exception: Any ideas to try?

I have been trying to solve a MashUp exception error for some time now. 

I have managed to simply the problem as follows. I have a main and related table, they have a common id between themselves. For debugging purposes the main table has only one column "id" and one row.

And I have a function GetRelated(id) that retrieves the related records from an API given an id. When I add a column with a call to this function referencing the id and then remove the column; DataFlow gives a MashUp Exception when published but looks fine in the PowerQuery Editor. Just a note: the error is the same if I dont remove the column.

Fail

let
    Source = mainTable,
    #"Invoked Custom Function" = Table.AddColumn(Source, "related", each GetRelated([id])),
  #"Removed columns" = Table.RemoveColumns(#"Invoked Custom Function", {"related"})
in
    #"Removed columns"

However if I retrieve the related records outside the failing query in another table...

GetRelatedInAdvance:

let
  Source = GetRelated("98035f6c-e16f-46cd-9fea-08db04f8aa2f")
in
  Source

And then add this table into a column

let
    Source = mainTable,
    #"Invoked Custom Function" = Table.AddColumn(Source, "related", each GetRelatedInAdvance),
  #"Removed columns" = Table.RemoveColumns(#"Invoked Custom Function", {"related"})
in
    #"Removed columns"

The flow works. Looks the same as the failing one in the editor.

What else should I look into?

Thanks

Ruchan