diff --git a/src/knx/tpuart_data_link_layer.cpp b/src/knx/tpuart_data_link_layer.cpp
index 1da2cf8..98f8c29 100644
--- a/src/knx/tpuart_data_link_layer.cpp
+++ b/src/knx/tpuart_data_link_layer.cpp
@@ -245,7 +245,7 @@ void TpUartDataLinkLayer::loop()
                 //Destination Address + payload available
                 _xorSum ^= rxByte;
                 //check if echo
-                if (_sendBuffer != NULL && (!((buffer[0] ^ _sendBuffer[0]) & ~0x20) && !memcmp(buffer + _convert + 1, _sendBuffer + 1, 5)))
+                if (_sendBuffer != nullptr && (!((buffer[0] ^ _sendBuffer[0]) & ~0x20) && !memcmp(buffer + _convert + 1, _sendBuffer + 1, 5)))
                 { //ignore repeated bit of control byte
                     _isEcho = true;
                 }