Cannot read property length of undefined

I am trying to simply check if I have an empty input text box but I get this error when I run this in Chrome:

Uncaught TypeError: Cannot read property 'length' of undefined.

Here is how I go about doing it. I check for DOM readiness and then call the function:

function walkmydog() {
    //when the user starts entering                                                                                                                                                
    if(document.getElementById('WallSearch').value.length == 0) {
        alert("nothing");
    }
}

if (document.addEventListener) {
    document.addEventListener("DOMContentLoaded", walkmydog, false);
}