diff --git a/libraries/SoftwareSerial/src/SoftwareSerial.cpp b/libraries/SoftwareSerial/src/SoftwareSerial.cpp
index 6a8c303c6c..20285fc6de 100644
--- a/libraries/SoftwareSerial/src/SoftwareSerial.cpp
+++ b/libraries/SoftwareSerial/src/SoftwareSerial.cpp
@@ -409,6 +409,8 @@ void SoftwareSerial::flush()
   noInterrupts();
   _receive_buffer_head = _receive_buffer_tail = 0;
   interrupts();
+  // wait for previous transmit to complete
+  while (active_out);
 }
 
 int SoftwareSerial::peek()