Unit Testing Node Script with chai-spy ... function spied on is undefined

Writing my first node.js test for my first gulp-plugin....using mocha and chai. I get TypeError: Cannot read property 'Assertion' of undefined for ar errorCheckArg = spy(errorCheckArg);. It seems the errorCheckArg function is not available in the testing enviroment(I did a console.log(errorCheckArg) and it showed undefined).

Any idea what I am doing wrong?

describe('gulp-foobar', function() {
    var fakeFile;

    beforeEach(function() {
        var fakeFile = new File({
            contents: new Buffer('abufferwiththiscontent')
        });
    });

    describe('get files', function() {

        it('should do something', function(done) {
            var foo = function() { };
            foobar(foo);
            var errorCheckArg = spy(errorCheckArg);
            expect(errorCheckArg).to.have.been.called.with(arguments);


            done();
        });
    });
});

---

being tested:

function foorbar(callback) {
    var destinationFile;
    errorCheckArg(arguments);
    return through2.obj(function(file, enc, next) {
    ...