geochat issue

gistfile1.txt

Uncaught exception: Error: Firebase.child failed: First argument was an invalid path: "null". Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]"

Error thrown at line 3, column 2 in g(a) in https://cdn.firebase.com/v0/firebase.js:
    throw a;
called from line 275, column 2 in Ia(a, b) in https://cdn.firebase.com/v0/firebase.js:
    (!u(b) || 0 === b.length || ya.test(b)) && g(Error(A(a, 1, l) + 'was an invalid path: "' + b + '". Paths must be non-empty strings and can\'t contain ".", "#", "$", "[", or "]"'))
called from line 3844, column 4 in <anonymous function: H.prototype.F>(a) in https://cdn.firebase.com/v0/firebase.js:
    Ia("Firebase.child", a);
called from line 106, column 2 in createUserNode(username) in https://dl.dropboxusercontent.com/u/1330446/geochat/js/scripts.js:
    userRef = allUserRef.child(username);
called from line 91, column 3 in successCallback(position) in https://dl.dropboxusercontent.com/u/1330446/geochat/js/scripts.js:
    createUserNode(username, marker);