Strictly speaking, it should be an adverb. Informal American English has fairly relaxed rules/conventions when it comes to adverb use, and it is seen as acceptable in colloquial US speech to use an adjective in place of an adverb. For example, a comment such as 'You did real good' is more acceptable in US English than it would be in British English. It's not 'right' as such, but it is acceptable. There is a historical reason for this. In fact, 'ly' adverbs only started to be widely used in the 18th century. Before that, it was quite common to have the same form of the word for both adjectival and adverbial uses. The word 'fast', which is both an adjective and an adverb, is a relic of that time. So are certain phrases such as 'Hold tight' and 'Take it easy'.