How do I make frame environment play nice with \textbf?

Question

I am getting a strange error message when I put textbf into a frame environment.

Minimal Example

\documentclass{article}
\listfiles

\begin{document}

\begin{frame}
\textbf{Lemma 1}

\end{frame}

\end{document}

Error Message:

! Missing number, treated as zero.
<to be read again>
                   }
l.8 \textbf
           {Lemma 1}

When I comment out the \textbf, the file compiles fine. When I use \textbf normally, the file also compiles fine.

Note: I am using pdflatex on Linux.

Answer 1

I'm not sure whether you want to use beamer's frame environment, or LaTeX's \frame command (which I haven't even known before:). If the former is the case, you're not loading the beamer class! If the latter: \frame is a command, not an environment. It takes only one argument: in your case, this will be a lone \textbf --- thus the error.

Thus, either:

\documentclass{beamer}

or:

\frame{\textbf{Lemma 1}}

Or, the third framed:

\usepackage{framed}
% ... 
\begin{framed}
  \textbf{Lemma 1}
\end{framed}

Answer 2

I'm not famliar with the frame environment. But the error seems to suggest that the frame environment takes some kind of argument and \textbf is getting slurped into that argument sans {Lemma 1}. This is why {\bf Lemma 1} appears to work.

Where did you run across \begin{frame}...\end{frame}?

\documentclass{article}
\listfiles
\usepackage{mdframed}
\pagestyle{empty}
\begin{document}

\begin{frame}{}
\textbf{Lemma 1}

\end{frame}

vs.

\begin{frame}{The}
\textbf{Lemma 1}

\end{frame}

\framebox{\textbf{Lemma 1}}

\end{document}