How to cite within mdframed using tufte-book

Question

I am using tufte-book and mdframed. When I try to use \cite inside the theorem environment I created with mdframed I get errors of the sort "Missing number, treated as zero...". Is there a way of getting around this problem. I tried using double {{}} as suggested here Cite in theorem environment argument but it does not work

\documentclass[a4paper,twoside,justified,titlepage]{tufte-book}
\usepackage{color}
\usepackage{mdframed}
\usepackage{mathtools,amssymb,latexsym,amsthm}
\newmdtheoremenv[%
backgroundcolor=myblue,%
]{theorem}{Theorem}
\newmdtheoremenv[%
backgroundcolor=myblue,%
]{definition}{Definition}

\definecolor{myblue}{rgb}{.8, .8, 1}
\begin{document}
        Hello~\cite{Homer}
\begin{theorem}[Derivation of the continuity equation]
    Hello~\cite{Homer}
\end{theorem}
\begin{proof}
    Testing
\end{proof}

\bibliographystyle{plain}
\bibliography{Bibfile}

\end{document}

@Book{Homer,
    Author = {Homer J. Simpson},
    Title = {Mmmmm...donuts},
    Year = {1999}
}