Does the align environment provided by amsmath not enter math mode?

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