  We present a HST WFC3 transmission spectrum for the transiting exoplanet
HAT-P-12b. This warm (1000 K) sub-Saturn-mass planet has a smaller mass and a
lower temperature than the hot-Jupiters that have been studied so far. We find
that the planet's measured transmission spectrum lacks the expected water
absorption feature for a hydrogen-dominated atmosphere, and is instead
best-described by a model with high-altitude clouds. Using a frequentist
hypothesis testing procedure, we can rule out a hydrogen-dominated cloud free
atmosphere to 4.9$\sigma$. When combined with other recent WFC3 studies, our
observations suggest that clouds may be common in exo-planetary atmospheres.
