Cannot convert value null to Table

I am able to refresh my Power BI report manually without any issue. However, when I schedule an automatic refresh to run overnight, it fails and I receive the following message. 

<ccon>Expression.Error: We cannot convert the value <ccon>null</ccon> to type <ccon>Table</ccon>.. </ccon>. The exception was raised by the IDbCommand interface. Table: Cards.

Still new to some of the language. What do I need to look for to hopefully resolve this?

Thanks!