diff --git a/src/crypto/c_blake256.c b/src/crypto/c_blake256.c
index 00a84c22..6aa497c1 100644
--- a/src/crypto/c_blake256.c
+++ b/src/crypto/c_blake256.c
@@ -134,7 +134,7 @@ void blake256_update(state *S, const uint8_t *data, uint64_t datalen) {
         if (S->t[0] == 0) S->t[1]++;
         blake256_compress(S, S->buf);
         data += fill;
-        datalen -= (fill << 3);
+        datalen -= (uint64_t)fill << (uint64_t)3;
         left = 0;
     }