This line contains the points (1, 0) and (1, 1). Its slope would be:

$\displaystyle\frac{{{y}_{{2}}-{y}_{{1}}}}{{{x}_{{2}}-{x}_{{1}}}}=\frac{{{1}-{0}}}{{{1}-{1}}}=\frac{{1}}{{0}}\usepackage{cmbright}\Huge\$

This is undefined.