Compiler says that hbar is invalid in math mode, but I can't figure out what I should be using instead -- all the sources say hbar?

The line is

$E = frac{hbar^2}{2I}J(J+1)$

and the error code is "LaTeX Warning: Command hbar invalid in math mode on input line 55."

Current packages: gensymb, amsmath, csquotes, placeins, cite, hanging, fontspec, graphicx, tikz, pgfplots