Plotting Exponentials Inside Logs in Latex pgfplots

I'm trying to add some plots to my document. I've been able to do so successfully before; but for some reason it's not working this time. I tried deleting everything except just one plot but I still get the same error. Let me just show the document with just the one plot.

\documentclass[11pt]{amsart}

\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
\begin{axis}[domain=0:3,samples=100,yticklabels=\empty]
\addplot+[no markers] {20-500*\ln(0.998*exp(-x)+0.002+0*exp(x))-500*\ln(0.998*exp(x)+0.002+0*exp(-x))};
\end{axis}
\end{tikzpicture}
\end{document}

I get the error "Undefined control sequence.\foreach ...reach\let \pgffor@assign@before@code=[lots more]"

When I replace the function with just the simple exp(x), the plot works. So what's wrong with this more complicated function?

Thanks

For the use in formula the logarithm function is ln not \ln

\documentclass[11pt]{amsart}

\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{pgfplots}

\begin{document}

\begin{tikzpicture}
\begin{axis}[domain=0:3,samples=100,yticklabels=\empty]
\addplot+[no markers] {20-500*ln(0.998*exp(-x)+0.002+0*exp(x))-500*ln(0.998*exp(x)+0.002+0*exp(-x))};
\end{axis}
\end{tikzpicture}
\end{document}