diff --git a/sim_ether.c b/sim_ether.c
index 0f5c369d..2d304d6b 100644
--- a/sim_ether.c
+++ b/sim_ether.c
@@ -1654,7 +1654,7 @@ static int _eth_get_system_id (char *buf, size_t buf_size)
   if ((status = RegOpenKeyExA (HKEY_LOCAL_MACHINE, "SOFTWARE\\Microsoft\\Cryptography", 0, KEY_QUERY_VALUE|KEY_WOW64_64KEY, &reghnd)) != ERROR_SUCCESS)
     return -1;
   reglen = buf_size;
-  if ((status = RegQueryValueExA (reghnd, "MachineGuid", NULL, &regtype, buf, &reglen)) != ERROR_SUCCESS) {
+  if ((status = RegQueryValueExA (reghnd, "MachineGuid", NULL, &regtype, (LPBYTE)buf, &reglen)) != ERROR_SUCCESS) {
     RegCloseKey (reghnd);
     return -1;
     }