“Dimension too large” error with XY-pic

What's wrong with the following?

\documentclass{amsart}
\usepackage[all]{xy}
\begin{document}
\xymatrix{
\text{Long string of text that causes an error during the compilation of this document} \\
X
}
\end{document}

Strangely if there's no second row it compiles fine.

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Welcome to TeX.sx! – Peter Jansson Mar 3 '13 at 6:36