Error executing an aggregate: cannot use non GROUP BY column '...' in query results without an aggregate function

[msiomkin]

An aggregate returns an error: cannot use non GROUP BY column '...' in query results without an aggregate function. But I don't have any columns in the SELECT list except my aggregate!

To Reproduce
Restore the database from the attached snapshot.
Create the aggregate using the attached source file.
Run it:

select myagg(bin1, value, bin2, bin3) from test;

Expected behavior

+-------------------------------------------------------------------------------------------+
| %1                                                                                        |
+===========================================================================================+
| 00010203                                                                                  |
+-------------------------------------------------------------------------------------------+

Actual behavior

SELECT: cannot use non GROUP BY column 'test.bin3' in query results without an aggregate function

Software versions

• MonetDB v11.47.3 (hg id: 79dbf838f044 88683)
• Almalinux 8
• self-installed and compiled

[njnes]

I have to do more debugging, but I guess the aggregator function somehow isn't found, causing this rather unclear error.