There was some defect in Tvastas chanting of the mantra because he chanted it long instead of short, and therefore the meaning changed. Tvasta intended to chant the word indra-satro, meaning, O enemy of Indra. In this mantra, the word indra is in the possessive case (sasthi), and the word indra-satro is called a tat-purusa compound (tatpurusa-samasa). Unfortunately, instead of chanting the mantra short, Tvasta chanted it long, and its meaning changed from the enemy of Indra to Indra, who is an enemy. Consequently instead of an enemy of Indras, there emerged the body of Vrtrasura, of whom Indra was the enemy.