diff --git a/src/knx/tpuart_data_link_layer.cpp b/src/knx/tpuart_data_link_layer.cpp
index 48fef44..f5505ec 100644
--- a/src/knx/tpuart_data_link_layer.cpp
+++ b/src/knx/tpuart_data_link_layer.cpp
@@ -235,13 +235,13 @@ void TpUartDataLinkLayer::loop()
                 //Destination Address + payload available
                 _xorSum ^= rxByte;
                 //check if echo
-                if (!((buffer[0] ^ _sendBuffer[0]) & ~0x20) && !memcmp(buffer + _convert + 1, _sendBuffer + 1, 5))
-                { //ignore repeated bit of control byte
-                    _isEcho = true;
-                }
-                else
+                _isEcho = false;
+                if(_sendBuffer != NULL)
                 {
-                    _isEcho = false;
+                    if (!((buffer[0] ^ _sendBuffer[0]) & ~0x20) && !memcmp(buffer + _convert + 1, _sendBuffer + 1, 5)) //ignore repeated bit of control byte
+                    {
+                        _isEcho = true;
+                    }
                 }
 
                 //convert into Extended.ind