Disclosed is a hemisulfate salt of Compound (I): and crystalline forms of the hemisulfate salt. Also disclosed are methods of using the hemisulfate salt of Compound (1) as a CGRP receptor antagonist, and pharmaceutical compositions comprising the hemisulfate salt of Compound (I). The hemisulfate salt of Compound (I) is useful in treating, preventing, or ameliorating disorders including migraine and other headaches, neurogenic vasodilation, neurogenic inflammation, thermal injury, circulatory shock, flushing associated with menopause, airway inflammatory diseases such as asthma, and chronic obstructive pulmonary disease (COPD).