diff --git a/mongoose.c b/mongoose.c
index 997053d6..e6271ae1 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -5800,7 +5800,7 @@ static size_t mkhdr(size_t len, int op, bool is_client, uint8_t *buf) {
   } else {
     uint32_t tmp;
     buf[1] = 127;
-    tmp = mg_htonl((uint32_t) (len >> 32));
+    tmp = mg_htonl((uint32_t) (((uint64_t) len) >> 32));
     memcpy(&buf[2], &tmp, sizeof(tmp));
     tmp = mg_htonl((uint32_t) (len & 0xffffffffU));
     memcpy(&buf[6], &tmp, sizeof(tmp));
diff --git a/src/ws.c b/src/ws.c
index 4eba7c5b..4a91acf9 100644
--- a/src/ws.c
+++ b/src/ws.c
@@ -110,7 +110,7 @@ static size_t mkhdr(size_t len, int op, bool is_client, uint8_t *buf) {
   } else {
     uint32_t tmp;
     buf[1] = 127;
-    tmp = mg_htonl((uint32_t) (len >> 32));
+    tmp = mg_htonl((uint32_t) (((uint64_t) len) >> 32));
     memcpy(&buf[2], &tmp, sizeof(tmp));
     tmp = mg_htonl((uint32_t) (len & 0xffffffffU));
     memcpy(&buf[6], &tmp, sizeof(tmp));