Central isotopes of $(-1,1)$-algebras

We prove that each $c$-isotope of a prime nonassociative $(-1,1)$-algebra is a $(-1,1)$-algebra if and only if the element $c$ lies in the commutative center. Every central $c$-isotope of a $(-1,1)$-monster is demonstrated to be isomorphic to the monster.