Profit measure isn't working, can't find solution

[KyleAdam]

Apologies for brief subject. I have a Profit and Loss report, and when I've only just noticed that the Profit measure that I'm using, which is simply just this:

[Invoices Paid] (sum of all invoices (Income)) - [PO Billed] (sum of all Purchase Orders (costs)) - [Hours Billed] (sum of all Hours Billed (costs)) 

That looked simple enough and it worked, until I filtered by the month and found a problem, here is the image below: 

If there is no invoice, then it is blank, rather than 0, so neither of the costs will be taken away. It does make sense. However, I can't think of a solution as to rectify this problem that I have. If anyone has any suggestions it will be most appreciated. 

NOTE: The three measures in profit [Invoices Paid], [PO Billed], [Hours Billed] are just sum of [insert column] measures and nothing more. They are simply just the total of a column and that's it, so for example [Invoices Paid] is this: 

Invoices Paid = SUM('Invoices (Books)'[Invoice Total]) 

[amitchandak · Accepted solution]

@KyleAdam , Based on what I got; try like

coalesce([Invoices Paid],0) - coalesce([PO Billed],0) - coalesce([Hours Billed],0)

[KyleAdam]

Thank you @amitchandak that worked a treat, I've not even heard of the coalesce, very useful.