Salesforce Flow : Check for null value in custom lookup field

Question

I have a flow that I'm triggering via Process Builder on the Opty Line Item object. The flow currently creates a new Opty and then updates a custom field on the original Opty Line Item with that new Opty's ID. The flow looks like this:

As you can see, I have a null check at the top of the flow for a field called CSM__c, which is located on the related Account to that Opty Line Item. If CSM is null, I want create the new Opty without referencing that CSM field. If it's not blank, then I'll pass the CSM value to the new Opty.

The issue I'm having is when triggering this flow with a null CSM__c field, SFDC throws this error:

The flow failed to access the value for ParentAccount.CSM__c because it hasn't been set or assigned.

I can't for the life of me figure out how to check if this field is null..my current null check looks like this:

I've also tried changing the formula to "EQUALS null", "EQUALS (blank)", "WAS SET false", using a formula for ISBLANK and ISNULL, literally all of those return the same error.

Would really appreciate if someone could share best practice for checking for null values in Flows.

EDIT I've updated my Flow and Decision screenshots above with the most recent versions.

Answer 1

When you pass a record from PB to Flow said record only contains fields that have values (not null). Otherwise those fields are "not set". You should check if they are not set first, then check if is null;

Use the was set operator and your conditions could look like this

{!ParentAccount.CSM__c}   was set   {!$GlobalCOnstant.FALSE}
{!ParentAccount.CSM__c}   is null   {!$GlobalCOnstant.TRUE}

with All conditions must be true (OR)

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Change your Decision Element to look like this:

Note that the only difference is this: