```
\documentclass[12pt]{article}

\usepackage{hyperref}

\usepackage{amsthm}
\usepackage{thmtools}

\usepackage{hyperref}

\theoremstyle{plain}

    \newtheorem{theorem}{Theorem}[section]

    \newtheorem{lemma}[theorem]{Lemma}

\begin{document}

\begin{lemma}
    \label{lemma:1.11}
    Lemma.
\end{lemma}

\begin{proof}[Proof of \texorpdfstring{\hyperref[lemma:1.1]{Lemma 1.1}}{Lemma 1.1}]
    Proof.
\end{proof}

\end{document}

```

This throws

```
Package hyperref Info: bookmark level for unknown lemma defaults to 0 on input
line 19.
! Argument of \Hy@babelnormalise has an extra }.

```

How can I fix this?

I noticed that removing the thmtools package solves the problem, but I need it, so that's not a valid solution.