Worth a read of the following thread.
Chances are your return values may contain invalid types.
Best thing to do is print the DB result before the return so we can see that’s causing the issue.
Basically, you’ll probably see things like Decimal('xxx') in there. You would need to convert those to float for example. There may also be others, but that one was the killer for me in that thread.
If you can post your output showing the result I can help identify the issue.