linking fields between a form and a subform fails - vba

Question

I'm stuck with an x-file trying to do something simple that I did thousands of times but now fails in a weird way.
I have a parent form called TASKS and a subform called TASKS_LIST. They are populated with recordsets that share some fields. The two important fields here are BUNDLE_ID (long) and TASK_ID (long).
Depending on certaing condition I want to link the subform with the parent form by the field BUNDLE_ID or by the field TASK_ID.
This is how I do it ->
First, in another form, I open the TASKS form like this:
DoCmd.OpenForm "TASKS", , , mVntCrit, , , intType
Where mVntCrit filters the records in the form and intType is an int with two possible values (1,2) passed by as an openArgs.
Then, on the Form_Open event of the parent form I do this:
Dim intTypeAs Integer
intType= Nz(OpenArgs)
Select Case intType
Case 1
Me.TASKS_LIST.LinkChildFields = "BUNDLE_ID"
Me.TASKS_LIST.LinkMasterFields = "BUNDLE_ID"
Case 2
Me.TASKS_LIST.LinkChildFields = "TASK_ID"
Me.TASKS_LIST.LinkMasterFields = "TASK_ID"
End Select
When I execute the code it appears an error message on the instructions that changes the linked fields:
Runtime Error '2101'
The specified value is not valid for this property.
I've tried/checked:
Initialize first the property values with an empty string to reset
the linking fields.
Change the order of the instructions, changing the value of the LinkMasterFields property first (althoug in principle the child goes first).
Compile, compact and repair.
Checked that the fields to link are the same type and have no nulls.
Checked that the fields name are correct.
What am I doing wrong? What am I missing?
Thanks a lot.

Answer 1

I found the issue.
I realized that the code worked for some records and failed for other records. So I needed to know what's different between the records that work and those than do not work.
Ok, in the original form that opens de TASKS parent form, there is a list of elements from a table T1. In the TASKS and TASKS_LIST forms the table T1 joins with a table T2 by a field F.
When I open the TASKS form, I filter its records with a certain criterion being F=Some Id from T1. The thing is: there are some records where this Id exists in T1 but not in T2. Therefore, when I open and filter the TASKS form with an Id that does not exists in T2 the form return no records. If TASKS do not return any records, TASKS_LIST do not return any records either.
In that case, when TASKS_LIST return no records, if I try to meddle with properties of the form (in this case .LinkChildFields and .LinkMasterFields) an error happens. On the other hand, when TASKS_LIST return some records, I can alter the form properties without problem.
Therefore I deduce that accessing and altering some properties of forms that return no records is forbidden for some reason. It reminds me of the classic error when trying to access the value of a texbox in a form without records.