Text box with dynamic value

I'm currently using a text box as a header with a dynamic value based off of my customer name. 

This works when I'm filtering on the customer name, it comes up: 

Sales Overview - Andrew
Sales Overview - Sarah 

etc. 

But if no customer name is selected it defaults to the first one, so with no filters it's: 
Sales Overview - Adam

Can this be altered? It's not a big issues but slightly fustrating... I'd like it to read 'Sales Overview - All', for example, not the top value.