diff --git a/tools/hwsim.c b/tools/hwsim.c
index 91b25574..c882abb5 100644
--- a/tools/hwsim.c
+++ b/tools/hwsim.c
@@ -260,7 +260,7 @@ static void hwsim_ready(void *user_data)
 			msg_size += 4;
 
 		if (radio_name_attr)
-			msg_size += sizeof(radio_name_attr);
+			msg_size += sizeof(radio_name_attr) + 1;
 
 		if (no_vif_attr)
 			msg_size += 4;