JSONPath evaluation problem

[AJW]

I've created a new item for MSSQL monitoring to get the alerts (different severities and the occurrence value
Being done as a discovery as need an item created per alert severity and their occurrence

Item - Database monitor
Select name as AlertName , occurrence_count as Occurrence from msdb.dbo.sysalerts where severity > 0

This produces currently the output (only one entry in the database at the moment)
[{"AlertName":"Error 17 Alert","Occurrence":"0"}]

Discovery created
select name as AlertName from msdb.dbo.sysalerts where severity > 0

Dependent item created using the discovery (Numeric(Unsigned)

Preprocessing created
$[?(@.AlertName == '{#ALERTNAME}')].Occurrance.first()

Produces the error;
Preprocessing failed for: 0
1. Failed: cannot extract value from json by path "$[?(@.AlertName=='Error 17 Alert')].Occurrence.first()": cannot parse as a valid JSON object: invalid object format, expected opening character '{' or '[' at: '0'

Now at a loss as to where the problem lies

Used the Job Discovery \ Get Job Status item as a template. The output from which looks in the same format as the one above (albeit with more entries returned)

Any one with any ideas where I'm going wrong ?
Any help greatly appreciated

[AJW]

Update: Just realised when typing here I put in a typo;
Preprocessing created
$[?(@.AlertName == '{#ALERTNAME}')].Occurrance.first()
should read
Preprocessing created
$[?(@.AlertName == '{#ALERTNAME}')].Occurrence.first()