diff --git a/lib/mpi/mpi.c b/lib/mpi/mpi.c
index 6bd8364..2d54be7 100644
--- a/lib/mpi/mpi.c
+++ b/lib/mpi/mpi.c
@@ -109,7 +109,7 @@ mpi_zero(cryb_mpi *X)
 
 	if (X->words == NULL) {
 		MPI_FAST_INIT(X);
-	} else {
+	} else if (X->msb != 0) {
 		memset(X->words, 0, X->size);
 		X->neg = 0;
 		X->msb = 0;