diff --git a/armsrc/iso14443a.c b/armsrc/iso14443a.c
index ef7b42415..f647b9ea8 100644
--- a/armsrc/iso14443a.c
+++ b/armsrc/iso14443a.c
@@ -2578,8 +2578,9 @@ void DetectNACKbug() {
 	uint8_t num_nacks = 0;
 	
 	#define PRNG_SEQUENCE_LENGTH	(1 << 16)
-	#define MAX_UNEXPECTED_RANDOM	4		// maximum number of unexpected (i.e. real) random numbers when trying to sync. Then give up.
-	#define MAX_SYNC_TRIES		32
+	#define MAX_UNEXPECTED_RANDOM	4		// maximum number of unexpected (i.e. real) random numbers when trying to sync, then give up.
+	#define MAX_SYNC_TRIES			32
+	#define MAX_PRNG_TRIES			20 		// when we gone through 10 prng sequences without managing to sync,  then give up.
 	
 	AppendCrc14443a(mf_auth, 2);
 	
@@ -2649,6 +2650,11 @@ void DetectNACKbug() {
 		// Transmit reader nonce with fake par
 		ReaderTransmitPar(mf_nr_ar, sizeof(mf_nr_ar), par, NULL);
 	
+		if ( elapsed_prng_sequences > MAX_PRNG_TRIES) {
+			isOK = -4; 			// Card's PRNG runs at an unexpected frequency or resets unexpectedly
+			break;
+		}
+	
 		// we didn't calibrate our clock yet,
 		// iceman: has to be calibrated every time.
 		if (previous_nt && !nt_attacked) { 
@@ -2682,6 +2688,11 @@ void DetectNACKbug() {
 				if (sync_cycles <= 0)
 					sync_cycles += PRNG_SEQUENCE_LENGTH;
 				
+				if (sync_cycles > PRNG_SEQUENCE_LENGTH * 2 ) {
+					isOK = -4; 			// Card's PRNG runs at an unexpected frequency or resets unexpectedly
+					break;
+				}
+				
 				if (MF_DBGLEVEL >= 4)
 					Dbprintf("calibrating in cycle %d. nt_distance=%d, elapsed_prng_sequences=%d, new sync_cycles: %d\n", i, nt_distance, elapsed_prng_sequences, sync_cycles);
 
diff --git a/client/cmdhfmf.c b/client/cmdhfmf.c
index ccb46901e..9fc12233b 100644
--- a/client/cmdhfmf.c
+++ b/client/cmdhfmf.c
@@ -2867,7 +2867,7 @@ int CmdHf14AMfNack(const char *Cmd) {
 		}
 	
 		if (WaitForResponseTimeout(CMD_ACK, &resp, 2000)) {
-			int32_t ok = resp.arg[0] & 0xff;
+			int32_t ok = resp.arg[0];
 			uint32_t nacks = resp.arg[1];
 			uint32_t auths = resp.arg[2];
 			
@@ -2880,7 +2880,14 @@ int CmdHf14AMfNack(const char *Cmd) {
 				case -4 : PrintAndLog("Card random number generator seems to be based on the wellknown");
 						  PrintAndLog("generating polynomial with 16 effective bits only, but shows unexpected behaviour."); return 1;
 				case  1 : PrintAndLog("Card has NACK bug."); return 1;
-				case  0 : PrintAndLog("Card may have NACK bug. inconclusive result"); return 1;
+				case  0 : {
+						if ( nacks > 0 ) 
+							PrintAndLog("Card may have NACK bug. inconclusive result"); 
+						else
+							PrintAndLog("Card has not NACK bug."); 
+						return 1;
+					}
+				default : PrintAndLog("  errorcode from device [%i]", ok); return 1;
 			}
 			break;
 		}