Uncaught TypeError: el.parent is not a function

[mamaluya]

var draw = SVG('drawing').size(500, 500);
var rect = draw.rect(100, 100);
rect.selectize();

nearly, I run the demo source on github, but met an error like below:
Uncaught TypeError: el.parent is not a function. where is my fault?

[Ulrich-Matthias Schäfer]

The code works as it should