diff --git a/sonoff/support_features.ino b/sonoff/support_features.ino
index 299f0aedd..21d67178b 100644
--- a/sonoff/support_features.ino
+++ b/sonoff/support_features.ino
@@ -376,7 +376,9 @@ void GetFeatures(void)
 #ifdef USE_MAX31855
   feature_sns2 |= 0x00080000;  // xsns_39_max31855.ino
 #endif
-//  feature_sns2 |= 0x00100000;
+#ifdef USE_PN532_I2C
+  feature_sns2 |= 0x00100000;  // xsns_40_pn532_i2c.ino
+#endif
 //  feature_sns2 |= 0x00200000;
 //  feature_sns2 |= 0x00400000;
 //  feature_sns2 |= 0x00800000;
diff --git a/sonoff/xsns_40_pn532_i2c.ino b/sonoff/xsns_40_pn532_i2c.ino
index ec6b959ab..656e7fff3 100644
--- a/sonoff/xsns_40_pn532_i2c.ino
+++ b/sonoff/xsns_40_pn532_i2c.ino
@@ -144,7 +144,7 @@ int16_t PN532_readResponse(uint8_t buf[], uint8_t len)
 }
 
 
-int8_t PN532_readAckFrame()
+int8_t PN532_readAckFrame(void)
 {
   const uint8_t PN532_ACK[] = {0, 0, 0xFF, 0, 0xFF, 0};
   uint8_t ackBuf[sizeof(PN532_ACK)];