Click a button to download a file using CasperJS

I want to write unit tests to download a file by clicking a form button using casperJS. I tried using document.getElementsByTagName to get to the button and clicked it in JavaScript's way. But I got an error message: TypeError: 'undefined' is not a function (evaluating 'inputs[1].click()')

The segment of the code that didn't pass the test :

casper.then(function() {
    var inputs = document.getElementsByTagName('input');
    inputs[1].click();
});

When I tried executing the same code from the console in my browser, the button got clicked and the download window popped up.

EDIT 1: I initially tried to check with the presence of the button with assertExists function in casperJS. The assertion passed, indicating that the button exists. Also, inputs array has 2 elements, one is hidden input of CSRF token (I'm using Django) and the other is the button that I want to click.