diff --git a/src/ext/ed25519/donna/modm-donna-64bit.h b/src/ext/ed25519/donna/modm-donna-64bit.h
index a47a38a42d..b22df2be75 100644
--- a/src/ext/ed25519/donna/modm-donna-64bit.h
+++ b/src/ext/ed25519/donna/modm-donna-64bit.h
@@ -240,7 +240,8 @@ contract256_window4_modm(signed char r[64], const bignum256modm in) {
 static void
 contract256_slidingwindow_modm(signed char r[256], const bignum256modm s, int windowsize) {
 	int i,j,k,b;
-	int m = (1 << (windowsize - 1)) - 1, soplen = 256;
+	int m = (1 << (windowsize - 1)) - 1;
+        const int soplen = 256;
 	signed char *bits = r;
 	bignum256modm_element_t v;