Applying \MakeUppercase on \makebox makes text disappear

Here is a MWE of the problem:

\documentclass{article}\begin{document}\makebox[4cm][l]{This is a test.}\MakeUppercase{\makebox[4cm][l]{This is a test.}}\end{document}

Which results in:

The logs don’t show any errors or warnings.

Why does this happen and how can I make the text in the second paragraph appear?