function sometime returns a NULL value

[Phong C]

I have the following query with a custom function. The get_test() function simply return the input variable. There are no complex calculations in the function.

Sometimes the query just return a NULL value while at other times it return the correct number(243). Not sure if anyone else is experiencing the same issue like this.


-------------
Query:
-------------
SELECT
dl.account_id AS 'accountId',
get_test(dl.account_id) AS 'test'
FROM document_line dl
JOIN document d
USING (document_id)
WHERE d.document_id = _document_id AND dl.line_amount != 0

-------------
Results:
-------------
"checks": [
{
"accountId": 243,
"test": null
},]

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Function:
-------------
CREATE FUNCTION get_test(
_account_id SMALLINT UNSIGNED
)
RETURNS SMALLINT UNSIGNED
DETERMINISTIC
BEGIN

RETURN _account_id;
END