You must be something

I know that in languages such as PIE adjectives are inflected for case, number, and gender of the noun to which they refer. But what if the noun is unspecified or even unknown? The situation I am thinking of is when it is used with an imperative of "to be": "Be afraid. Be very afraid." Are we to attempt to determine the gender and number of the person who must be fearful? And in any case, what would be the case?

David Fickett-Wilbar