Panel + SQLAlchemy issue

hi, I am using Panel for a few dashboards at my company. The dashboards work well but after a while I get the following error and have to stop and restart the process:

Exception in callback functools.partial(<bound method IOLoop._discard_future_result of

<tornado.platform.asyncio.AsyncIOMainLoop object at 0x00000127F6D81B88>>,

<Future finished exception=KeyError('value_time')>). 

The data displayed in the dashboard is coming from a sql server database which I access through SQLalchemy. “value_time” is a field in my data. Any idea of what the error could be?