[SalesForce] What Criteria to use in Process Builder to check for NULL reference

I have a lookup field on account – for other accounts – Name: Account Parent Tiered OBJ

How do I check in Process Builder that it's actually empty?

Using the Global Constant NULL

or: IS Null true

both returned the wrong answer [there always seem to be some value there]

• As debug, I tried to get the id/name/any value form the 'so called not null obj', but I get error:

The record couldn’t be saved because it failed to trigger a flow. A flow trigger failed to execute the flow with

So yea, it is NULL, but I just can't understand how to check for it.

• My process

• Error report:

 An error occurred at element myRule_1_A1 (FlowRecordUpdate).
The flow failed to access the value for myVariable_current.Account_Parent_tiered_Obj__r.Name because it hasn't

been set or assigned.

This report lists the elements that the flow interview executed. The report is a beta feature.
We welcome your feedback on IdeaExchange.

Flow Details
Flow Name: test_account_parent_obj
Type: Workflow
Version: 3
Status: Active

Flow Interview Details
Interview Label: test_account_parent_obj-3_Account
Current User: Saar Machtinger (00520000000vhNm)
Start time: 12:54 11/01/2016
Duration: 0 seconds

How the Interview Started
Saar Machtinger (00520000000vhNm) started the flow interview.
Some of this flow's variables were set when the interview started.
myVariable_old = 00120000006zEWSAA2
myVariable_current = 00120000006zEWSAA2
RecursiveCountVariable = 0.00

ASSIGNMENT: myVariable_waitStartTimeAssignment
{!myVariable_waitStartTimeVariable} Equals {!Flow.CurrentDateTime}
Result
{!myVariable_waitStartTimeVariable} = "12:54 11/01/2016"

DECISION: isChangedDecision2_myRule_1_ParentId
Executed this outcome: isChangedRule_2_myRule_1_ParentId
Outcome conditions: and
1. {!myVariable_old} (00120000006zEWSAA2) Is null false
2. {!myVariable_old.ParentId} (null) Does not equal {!myVariable_current.ParentId} (00120000003rB3xAAE)
Logic: All conditions must be true (AND)

DECISION: isChangedDecision4_myRule_3_ParentId
Executed this outcome: isChangedRule_4_myRule_3_ParentId
Outcome conditions: and
1. {!myVariable_old} (00120000006zEWSAA2) Is null false
2. {!myVariable_old.ParentId} (null) Does not equal {!myVariable_current.ParentId} (00120000003rB3xAAE)
Logic: All conditions must be true (AND)

DECISION: myDecision
Executed this outcome: myRule_1
Outcome conditions: and
1. {!isChangedRule_2_myRule_1_ParentId} (true) Equals true
2. {!myVariable_current.ParentId} (00120000003rB3xAAE) Is null false
3. {!myVariable_current.Account_Parent_tiered_Obj__c} (null) Is null true
Logic: All conditions must be true (AND)

NOTE: I do not have a default value on the field.

• no field udpates or flows using this field