Horizontal line in alignat* in beamer with overlays

Here is the MWE:

\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\begin{frame}[shrink]
\begin{alignat*}{5}
\uncover<+->{t_n &= a + (n-1)d  \\}
\uncover<+->{t_3 &= a + (3-1)d \tag{1} \\}
\cline{1-1}
\uncover<+->{t_3 &= a + (3-1)d  \\}
\uncover<+->{t_3 &= a + (3-1)d  \\}
\uncover<+->{-20 &= -20 \tag{2}\\}
\notag
\end{alignat*}
\vskip-1.5em
\end{frame}
\end{document}


With \cline, it produces a cryptic error:

! Misplaced \omit.
\@cline #1-#2\@nil ->\omit
\@multicnt #1\advance \@multispan \m@ne \ifnum \@...
l.16 \end{frame}


How do I get a simple horizontal line in the place indicated? It would be great if it can have an overlay.

Do you want this?

\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\begin{frame}[shrink]
\begin{alignat*}{5}
\uncover<+->{t_n &= a + (n-1)d  \\}
\uncover<+->{t_3 &= a + (3-1)d \tag{1} \\\cline{1-1}}
\uncover<+->{t_3 &= a + (3-1)d  \\}
\uncover<+->{t_3 &= a + (3-1)d  \\}
\uncover<+->{-20 &= -20 \tag{2}\\}
\notag
\end{alignat*}
\vskip-1.5em
\end{frame}
\end{document}


Edit: Or this?

\documentclass{beamer}
\usepackage{amsmath}
\begin{document}
\begin{frame}[shrink]
\begin{alignat*}{5}
\uncover<+->{t_n &= a + (n-1)d  \\}
\uncover<+->{t_3 &= a + (3-1)d \tag{1}}
\uncover<1->{ \\\cline{1-1}}
\uncover<+->{t_3 &= a + (3-1)d  \\}
\uncover<+->{t_3 &= a + (3-1)d  \\}
\uncover<+->{-20 &= -20 \tag{2}\\}
\notag
\end{alignat*}
\vskip-1.5em
\end{frame}
\end{document}