Can't MERGE with null values; 'Cannot merge node using null property value' in neo4j

Question

I have a column in a csv that looks like this:

I am using this code to test how the splitting of the dates is working:

LOAD CSV WITH HEADERS FROM
'file:///..some_csv.csv' AS line
WITH
SPLIT(line.date_of_birth, '/') AS date_of_birth
return date_of_birth;

This code block works fine and gives me what I'd expect, which is a collection of three values for each date, or perhaps a null if there was no date ( e.g,

[4, 5, 1971]  
[0, 0, 2003]  
[0, 0, 2005]  
 . . .  
null  
null  
 . . .  

My question is, what is this problem with the nulls that are created, and why can't I do a MERGE when there are nulls?

LOAD CSV WITH HEADERS FROM
'file:///..some_csv.csv' AS line
WITH
SPLIT(line.date_of_birth, '/') AS date_of_birth, line
MERGE (p:Person {
 date_of_birth: date_of_birth
});

This block above gives me the error:

Cannot merge node using null property value for date_of_birth  

I have searched around and have only found one other SO question about this error, which has no answer. Other searches didn't help.

I was under the impression that if there isn't a value, then Neo4j simply doesn't create the element.

I figured maybe the node can't be generated since, after all, how can a node be generated if there is no value to generate it from? So, since I know there are no ID's missing, maybe I could MERGE with ID and date, so Neo4j always sees a value.

But this code didn't fare any better (same error message):

LOAD CSV WITH HEADERS FROM
'file:///..some_csv.csv' AS line
WITH
SPLIT(line.date_of_birth, '/') AS date_of_birth, line
MERGE (p:Person {
 ID: line.ID
,date_of_birth: date_of_birth
});  

My next idea is that maybe this error is because I'm trying to split a null value on slashes? Maybe the whole issue is due to the SPLIT.

But alas, same error when simplified to this:

LOAD CSV WITH HEADERS FROM
'file:///..some_csv.csv' AS line
WITH line
MERGE (p:Person {
 subject_person_id: line.subject_person_id
,date_of_birth: line.date_of_birth
});

So I don't really understand the cause of the error. Thanks for looking at this.

EDIT

Both @stdob-- and @cybersam have both answered with equally excellent responses, if you came here via Google please consider them as if both were accepted

Answer 1

As @cybersam said merge not work well with queries where the properties are set within the scope in null. So, you can use on create and on match:

LOAD CSV WITH HEADERS FROM
  'file:///..some_csv.csv' AS line
MERGE (p:Person {
  subject_person_id: line.subject_person_id
})
  ON CREATE SET p.date_of_birth = line.date_of_birth
  ON MATCH SET p.date_of_birth = line.date_of_birth