Count rows of a filtered tablix

I have a tablix and I want to show in a textbox outside of the tablix the number of rows.

With CountRows("customers") I get the result.

However, I have a parameter that users can set to filter the result. The parameter is included in the tablix's filters.

CountRows("customers") always return number of rows of the dataset, not the filtered tablix.

How can I get the number of rows of a filtered tablix?