David Letterman showed up for work, but had to go home sick Tuesday before taping the "Late Show." A stomach bug was to blame, a network spokeswoman said. Adam Sandler, one of the night's guests, was quickly enlisted to fill in as host. The CBS late-night personality has had extended absences following heart surgery and a case of the shingles. But this was believed to be the first time Letterman showed up for work and could not go on.