Why does this give me the Missing number, treated as zero error

I have been trying to type $\hat{A}=(2/\hbar)\boldsymbol{a}\cdot\boldsymbol{\hat{S}}$ into miktex yet it keeps giving me the missing number error. Can someone please explain how I can fix this?

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• As long as I load the amsmath package, it works for me: \documentclass{article} \usepackage{amsmath} \begin{document} $\hat{A}=(2/\hbar)\boldsymbol{a}\cdot\boldsymbol{\hat{S}}$ \end{document} – Steven B. Segletes Apr 6 at 0:52