Mathjax: bbox and align?

I’m trying to use \bbox to enclose an expression in a box. However, when doing so within an {align} block and using the & alignment operator, it returns an error:

$$
\begin{align}
\bbox[5px, border: 2px solid red]{f'(x)&=\dfrac{1}{g'(f(x))}} \\\\
\end{align}
$$

image

{for context, this is how it looks without the alignment operator block:

$$
\begin{align}
\bbox[5px, border: 2px solid red]{f'(x)=\dfrac{1}{g'(f(x))}} \\\\
\end{align}
$$

image

Any ideas if I’m doing something wrong or if it’s a bug?

It’s just LaTeX problem.

Solution

(There is \boxed instead of \bbox, but method is the same.)