How to correctly throw an TypeError when certain parameters are used in a function using Mocha/Chai

So, I got this code:

function initial(a,b,c){
    if ( isNaN(a) || isNaN(b) || isNaN(c) ){
        //do something
    }
    else {
        //do something
    }  
}

I need to expect an TypeError when a string is used instead of a number. Initially I thought that declaring expectations for a, b and c separately would be ok, like this:

 expect(a).to.be.a('number');
 expect(b).to.be.a('number');
 expect(c).to.be.a('number');

I want it to actually observe the function and expect certain parameters to throw an error, something like:

expect( initial('foo','foo','foo') ).to.Throw(TypeError);

But this is not working, and I've tested quite some options and none seem to actual give me the TypeError I want. Does any body know the right way to expect certain function parameters?

Thank you!