# Cross Reference Enumeration from a Different Document

I'm working on a Design document that is based on a Requirements document that is subject to change. The Design document must reference Requirements in an enumerated list from the Requirements Document.

My requirements document has something like this:

\documentclass{article}
\usepackage{graphicx}
\usepackage{mathtools}
\begin{document}
\begin{enumerate}
\item Requirement 1  \label{Req1}
\item Requirement 2:
\begin{enumerate}
\item Requirement 2a \label{Req2a}
\item Requirement 2b \label{Req2b}
\end{enumerate}
\end{enumerate}
\end{document}

My design document looks like this:

\usepackage{graphicx}
\usepackage{geometry}
\usepackage{fancyhdr}
\usepackage{amsmath, amsthm ,amssymb}
\usepackage{graphicx}
\usepackage{enumerate}
\usepackage{hyperref}
\usepackage{xr}

\begin{document}
\externaldocument{../requirements/Requirements.tex}

\section{Functional Requirements}
Functional Requirements are things the system must do. These are: \\

\begin{tabular}{| l | l |}
\hline
Requirement & Subsystem(s) implementing the requirement \\ \hline
\ref{Req1} & Graphical Front End \\ \hline

\end{tabular}

\end{document}

I'd like to see the number that is given to the item in the enumeration show up in the \ref{Req1} space. But what I see now are two question marks. What am I missing?