Format A String Of Text From A Variable?

I seem to be having a problem with the Format() Function. I am trying to format a string of text from a variable so that it uses commas as thousand separators, but I can't seem to get it working properly. Here's what I'm using:

Dim testVar As String
testVar = "10000"
testVar = Format$(testVar,

When I use this code, it changes the variable's value to The strange thing is, it works fine if I use a number instead of the Variable. It also works fine if I use a "predefined numeric format" instead of my custom one. For example, if I use:

testVar = Format$(testVar, "Standard")it works just fine, but that's not what I want (otherwise I'd just do it this way.) It also works fine when I do this:

testVar = Format$(10000, And, although in the example code I've written here, it seems like I could just do it this way, in my actual program, I can't. I have to set it dynamically with the variable.

So it seems like both portions of my Format() Function are working, but just not together, and I can't seem to figure out why. anyone shed some light on this for me and tell me what I'm doing wrong? From every example I can find on the internet about using this function with a variable, I'm doing it correctly, but mine isn't working.