diff --git a/libraries/SoftwareSerial/src/SoftwareSerial.cpp b/libraries/SoftwareSerial/src/SoftwareSerial.cpp
index 3163d7a31..5a387ab94 100644
--- a/libraries/SoftwareSerial/src/SoftwareSerial.cpp
+++ b/libraries/SoftwareSerial/src/SoftwareSerial.cpp
@@ -409,7 +409,7 @@ int SoftwareSerial::available()
   if (!isListening())
     return 0;
 
-  return (_receive_buffer_tail + _SS_MAX_RX_BUFF - _receive_buffer_head) % _SS_MAX_RX_BUFF;
+  return ((unsigned int)(_receive_buffer_tail + _SS_MAX_RX_BUFF - _receive_buffer_head)) % _SS_MAX_RX_BUFF;
 }
 
 size_t SoftwareSerial::write(uint8_t b)