Tabularx that includes math won't compile for rows containing exponents

I have a tabularx table that will not compile unless I comment out all the rows that contain exponents. I must be missing a typo, right?

Here is the most I could reduce it:

%!TEX encoding = UTF-8 Unicode 
documentclass[12pt]{article}
usepackage{amsmath}
usepackage{tabularx}

begin{document}

begin{tabularx}{textwidth}{ l | X }
hline  
multicolumn{2}{l}{stuff} 
hline
texttt{x: array(1..h, 1..n, 1..m) of mpvar} & (x) 
texttt{y_r: array(1..h, 1..n, 1..(m-1)) of mpvar} & (y^r )   % Compiles if you comment out these lines
texttt{y_d: array(1..h, 1..(n-1), 1..m) of mpvar} &  (y^d )    %
end{tabularx}
 
end{document} 

The same behavior occurs if I use dollar signs instead of parentheses.