Filter Sum in Access Form

[TheWebDataGuy]

I am using a filter in one of my forms. The filter criteria comes from a listbox.

In the form footer I want the sum of the filtered records to be displayed.

Using '=SUM([ColumnName])' results in the SUM of ALL the records and not just the filtered ones which is what I want.
And when I switch between the filter criterias, it gives me an error in the sum field.

Any ideas on how to sum up filtered data on a form?

Thanks!