Calling macro fails with error \inaccessible

I am trying to define a macro which I will include in a class definition. For the time being I put the macro definition in the normal document. Compiling this document fails:

\documentclass[10pt,a4paper]{article}

\makeatletter
\def\ntcstitle#1{\def{\@ntcstitle}{#1}}
\newcommand{\thentcstitle}{\@ntcstitle}
\makeatother

\ntcstitle{Lorem Ipsum}

\begin{document}
\thentcstitle\par
\end{document}

This example is something I copied from another question here, which seemed to work: Define variable in a class

I also tried to move this definition into a class file. Omitted the \makeatletter and \makeatother commands. Using the class file was successful, the error message is the same.

This is the error message:

! Missing control sequence inserted.
<inserted text> 
                \inaccessible 
l.10 \ntcstitle{Lorem Ipsum}

Then I replaced the macro with the Latex style definition:

\documentclass[10pt,a4paper]{article}
\usepackage{lipsum}

\makeatletter
\newcommand{\ntcstitle}[1]{\newcommand{\@ntcstitle}{#1}}
\newcommand{\thentcstitle}{\@ntcstitle}
\makeatother

\ntcstitle{Lorem Ipsum}

\begin{document}
\thentcstitle\par
\end{document}

And that works as expected.

I tried to use \def instead of \newcommand because I assumed \def was the correct way to define macros in class files. What I am doing wrong here?