'When a man being asleep, reposing, and at perfect rest sees no dreams, that is the Self, this is the immortal, the fearless, this is Brahman.' Then Indra went away satisfied in his heart. But before he had returned to the Devas, he saw this difficulty. In truth he thus does not know himself (his self) that he is I, nor does he know anything that exists. He is gone to utter annihilation. I see no good in this.

In plain words: Prajapati now says the Self is what remains in deep, dreamless sleep. Indra leaves satisfied but then worries: in deep sleep a person does not know "I am this," nor anything else, so it seems like being wiped out entirely.