Wrong sample code for fastsin2 in Tutorial 4 of 5?

[CounterMatter]

I really like this tool, so I was playing around with making a GUI for it (e.g. to easily see the error graph). I was working through the tutorials in the wiki section, and I noticed that Tutorial 4 of 5 (https://github.com/samhocevar/lolremez/wiki/Tutorial-4-of-5:-fixing-lower-order-parameters) includes this code:

float fastsin2(float x)
{
    return x + x * f(x * x);
}

Graphing that doesn't giving me the correct results. I worked through the math, and I believe the code should be updated to:

float fastsin2(float x)
{
    return x + x * x * x * f(x * x);
}

(It was missing a factor of x^2.)

Am I overlooking something?