Beamer: code fragment with listings

Question

I am having difficulty adding code fragment to my slides in beamer with listings package.

My code:

\documentclass{beamer}
%\usepackage[utf8]{inputenc}
\usetheme{Warsaw}
\usepackage{listings}
\title{My slies}
\author{Me }
\date{October 2020}
%slide number
\expandafter\def\expandafter\insertshorttitle\expandafter{%
  \insertshorttitle\hfill%
  \insertframenumber,/,\inserttotalframenumber}
\begin{document}
\begin{frame}{Slide 1}
\begin{lstlisting}[basicstyle=\footnotesize]
def LosScore(y_pred, y_test):
  mse = []
  for i in range(len(y_test)):
     mse.append(np.mean(np.square((y_test[i]-y_pred[i]))))
return mse
\end{lstlisting}
\end{frame}
\end{document}

Error:

Runaway argument?
! Paragraph ended before \lst@next was complete.
<to be read again> 
                   \par 
l.27 \end{frame}
I suspect you've forgotten a `}', causing me to apply this
control sequence to too much text. How can we recover?
My plan is to forget the whole thing and hope for the best.