diff --git a/client/mifare4.c b/client/mifare4.c
index cf6c8f42..8e363779 100644
--- a/client/mifare4.c
+++ b/client/mifare4.c
@@ -17,10 +17,11 @@
 #include "polarssl/libpcrypto.h"
 
 int CalulateMAC(mf4Session *session, uint8_t *data, int datalen, uint8_t *mac, bool verbose) {
-	memset(mac, 0x00, 8);
 	if (!session || !session->Authenticated || !mac || !data || !datalen)
 		return 1;
 	
+	memset(mac, 0x00, 8);
+	
 	if (verbose)
 		PrintAndLog("MAC data[%d]: %s", datalen, sprint_hex(data, datalen));