diff --git a/src/simutil.c b/src/simutil.c
index 6e7051ed..d5fc06bf 100644
--- a/src/simutil.c
+++ b/src/simutil.c
@@ -1628,9 +1628,10 @@ static int build_authenticate(unsigned char *buffer, const unsigned char *rand,
 		buffer[pos++] = 0x10;
 		memcpy(buffer + pos, autn, 16);
 		pos += 16;
-		buffer[pos++] = 0x00;
 	}
 
+	buffer[pos++] = 0x00;
+
 	return pos;
 }