Cross-referencing with amsthm and cleveref [closed]

Question

I have to reference lemmas and theorems I have created using amsthm, but I want the actual word (e.g"Lemma" or "Theorem") to show up whenever I reference them. \ref didn't do the trick, as that only gave me a number. After some googling I came across cleveref, which is supposed to solve it. Unfortunately, whenever I typeset my document, it gives me two question marks instead of the name of the environment I'm trying to reference (e.g. instead of getting "Lemma 7.3" I get "?? 7.3). Here's a MWE:

\documentclass[12pt]{article}
\usepackage{amsmath,amsthm}
\usepackage{cleveref}
\theoremstyle{definition}
    \newtheorem{defn}{Definition}[section]
\theoremstyle{plain}
    \newtheorem{thm}[defn]{Theorem}
\newtheorem{lemma}[defn]{Lemma}
    \newtheorem{cor}[defn]{Corollary}

\crefname{lemma}{lemma}{lemma} 
\Crefname{lemma}{Lemma}{Lemmas}


\begin{document}
\begin{lemma}[My lemma]\label{lemma1}
\end{lemma}

Hence, by \cref{lemma1}

\end{document}

Is there something I'm doing wrong here?