Why are the numbers (acually, letters) for the sub-captions missing in the following MWE? note the colon that would follow the letter is printed.
\documentclass{tufte-handout}
\usepackage{tikz}
\usepackage{caption}
\usepackage{subcaption}
\begin{document}
\begin{figure}
\begin{subfigure}[b]{0.3\textwidth}
\centering
\begin{tikzpicture}
\draw circle (1.25cm) {};
\end{tikzpicture}%
\caption{A circle, bottom-aligned}
\label{fig:circle}
\end{subfigure}%
~
\begin{subfigure}[b]{0.3\textwidth}
\centering
\begin{tikzpicture}
\draw circle (1cm) {};
\end{tikzpicture}%
\caption{A second, smaller circle}
\label{fig:circle}
\end{subfigure}%
~
\begin{subfigure}[b]{0.3\textwidth}
\centering
\begin{tikzpicture}
\draw circle (1.25cm) {};
\end{tikzpicture}%
\caption{A third circle, with a long caption that will force more
line breaks and mess up the pretty layout}
\label{fig:circle3}
\end{subfigure}%
\\
\begin{subfigure}[b]{0.3\textwidth}
\centering
\begin{tikzpicture}
\draw circle (1.25cm) {};
\end{tikzpicture}%
\caption{another circle, in a new row}
\label{fig:circle4}
\end{subfigure}%
~
\begin{subfigure}[b]{0.3\textwidth}
\centering
\begin{tikzpicture}
\draw circle (1.25cm) {};
\end{tikzpicture}%
\caption{final circle}
\label{fig:circle5}
\end{subfigure}%
\caption{Shapes with no corners. Ugly sub-figure layout. Caption for
the figure is in the margin per the \texttt{tufte-handout}
class}\label{fig:circles}
\end{figure}
\end{document}