Two different averages in one table

Hi,

I have a little problem.

I have one table with costumors and there is recorded how the invoiceprocess goes.

I have another table with invoices and in there I have a colomn saying how fast after the order the invoice was sent.

Now I have a table that gives me from all costumers what there average is on how fast the invoice was send after the order.

I have a differente table that gives me from all invoiceprocesses the average on how fast the invoice was send after the order.

This second number I want to use in the first table to show the difference.

I have used this: 

Verschil = CALCULATE(AVERAGE('dba KO'[Datumverschil]);ALLEXCEPT('dba Relatie';'dba Relatie'[FacturatieWijzeCode]))

 

But then the table changes showing me rows without values, because they have a invoiceprocess but no invoice.

And because they dont have a invoice there is no average.