[Tex/LaTex] Does the align environment provided by amsmath not enter math mode

amsmath

I asked a question earlier today, where I learned that the subequations environment provided by amsmath does not enter math mode: Missing $ inserted error when using \frac inside subequations environment?

According to the documentation however, align should enter math mode (see pg. 10). Then, why does the following MNWE (minimum not working example) not work?

\documentclass[12pt,letterpaper]{article}

\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}

\section{Hello World}
    \begin{subequations}
        \begin{align}
            \frac{a}{b} &= 0 \\

            x + y &= \frac{a}{b} \\
        \end{align}
    \end{subequations}

\end{document}

The errors being produced seemingly suggesting once again that align is not a math mode environment?

line 14: Missing $ inserted I've inserted a begin-math/end-math symbol since I think
line 15: Missing \endgroup inserted x + y &
line 15: Missing $ inserted x + y &= \frac{a}{b}
line 15: Extra }, or forgotten $ x + y &= \frac{a}{b}
line 16: Misplaced \cr \end{align}
line 16: Missing $ inserted \end{align}
line 16: Missing } inserted \end{align}
line 16: Misplaced \noalign \end{align}
line 16: Extra }, or forgotten \endgroup \end{align}
line 16: Missing $ inserted \end{align}
line 0: Display math should end with $$

In particular, note the error for line 14: "I think therefore I insert $?". Okay, all jokes aside, what's up here? Am I not understanding the documentation correctly?

Best Answer

This is even more than in your answer (now comment). In any form of math mode, \par and equivalent forms, like an empty line, causes an error. The reason is in the Chapter 16, p. 135 of The TeXbook

Conversely, a blank line or \par is not permitted in math mode. This gives TeX another way to recover from a missing $; such errors will be confined to the paragraph in which they occur.