Question : Aggregate Function

I am new to Access but I have managed to create a query from several tables and it runs quite well.  I tried to add a formula to my query based on elements of the tables I am using and I get the message "You tried to execute a query that does not include the specified expression 'CustomerID' as part of an aggregate function.  I have no idea what this means, so I have no idea how to fix this.

Thanks,

Answer : Aggregate Function

in query designer when you put in "totals" you have to select an aggregate for each item.  The drop down includes sum mac avg count min, etc.  It also includes "expression".  The formula field needs to be set to expression.