diff --git a/crypto/ecc.c b/crypto/ecc.c
index 8914e437f94b8bc136b93206b32b89172514a24f..d15ef0747a7679ed0b66ad43d41d8768d01db407 100644
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -1320,7 +1320,10 @@ static void ecc_point_mult(struct ecc_point *result,
 	carry = vli_add(sk[0], scalar, curve->n, ndigits);
 	vli_add(sk[1], sk[0], curve->n, ndigits);
 	scalar = sk[!carry];
-	num_bits = sizeof(u64) * ndigits * 8 + 1;
+	if (curve->nbits == 521)	/* NIST P521 */
+		num_bits = curve->nbits + 2;
+	else
+		num_bits = sizeof(u64) * ndigits * 8 + 1;
 
 	vli_set(rx[1], point->x, ndigits);
 	vli_set(ry[1], point->y, ndigits);