Why does adding a \Bar in a differential equation cause all these errors?

The line in question:

B &= \diffp* { f(x, \Bar{\mu}) } { \Bar{\mu} } { x_{eq}, \Bar{\mu}_{eq} }

The second \Bar{\mu} (bolded for clarity) seems to throw everything awry. Omitting the \Bar (just having '{ \mu }') works just fine, everything compiles with no errors and displays as expected. Including the \Bar gives me 44 errors, the first few of which would suggest I forgot a '}', but I've checked and triple checked, and I can find no missing or mismatched brackets.

I'm guessing there's some quirk of \diffp* that's causing this, but it's what I've used everywhere else in the document, and I'd rather not have to go back and change them all to \frac for consistency. So how do I get a bar on the bottom with \diffp*, and why is my way incorrect?

EDIT: \diffp* comes from the package esdiff.

A full example doc:

\documentclass[12pt]{extarticle}
\usepackage[utf8]{inputenc}
\usepackage{cite}
\usepackage{amsmath}
\usepackage{esdiff}
\usepackage{ amssymb }

\begin{document}

\begin{align*}
A &= \diffp*{f(x,\bar{\mu})}{x}{x_{eq},\Bar{\mu}_{eq}}\\
B &= \diffp* {f(x, \Bar{\mu})} {\Bar{\mu}} {x_{eq},\Bar{\mu}_{eq}}
\end{align*}

\end{document}
• What is \Bar? Where did you get it from? What package? Did you mean \bar? – Steven B. Segletes Feb 17 '19 at 17:36
• Definitely meant \Bar. I'm pretty sure it's from amsmath, could be amssymb. I should probably also mention that this line is in an align* block with a similar line above it. – Nick J Feb 17 '19 at 17:44
• After double checking, \bar and \Bar seem to do the same thing. Both add a bar no top of the symbol, both throw the same errors in that line. – Nick J Feb 17 '19 at 17:47
• Why don't you give us a small, but complete document that displays the issue? – Steven B. Segletes Feb 17 '19 at 17:51
• \documentclass[12pt]{extarticle} \usepackage[utf8]{inputenc} \usepackage{cite} \usepackage{amsmath} \usepackage{esdiff} \usepackage{ amssymb } \begin{document} \begin{align*} A &= \diffp*{f(x,\bar{\mu})}{x}{x_{eq},\Bar{\mu}_{eq}}\\ B &= \diffp* {f(x, \Bar{\mu})} {\Bar{\mu}} {x_{eq},\Bar{\mu}_{eq}} \end{align*} \end{document} – Nick J Feb 17 '19 at 17:54

You seem to have missed out the {} of one argument, but some failing in the \diffp macro requires it to be doubled.

\documentclass[12pt]{extarticle}
\usepackage[utf8]{inputenc}
\usepackage{cite}
\usepackage{amsmath}
\usepackage{esdiff}
\usepackage{ amssymb }

\begin{document}

\begin{align*}
A &= \diffp*{f(x,\bar{\mu})}{x}{x_{\mathrm{eq}},\Bar{\mu}_{\mathrm{eq}}}\\
B &= \diffp* {f(x, \Bar{\mu})} {{\Bar{\mu}}} {x_{\mathrm{eq}},\Bar{\mu}_{\mathrm{eq}}}
\end{align*}

\end{document}

The problem comes from the second equation B &= \diffp* {f(x, \Bar{\mu})} {\Bar{\mu}} {x_{eq},\Bar{\mu}_{eq}}. The second argument \Bar{\mu} should be in parenthesis, ie, {\Bar{\mu}}. So, second equation should be B &= \diffp* {f(x, \Bar{\mu})} {{\Bar{\mu}}} {x_{eq},\Bar{\mu}_{eq}}