diff --git a/libraries/USBDevice/USBSerial/CircBuffer.h b/libraries/USBDevice/USBSerial/CircBuffer.h
index ea46bdfe6f..654c370c57 100644
--- a/libraries/USBDevice/USBSerial/CircBuffer.h
+++ b/libraries/USBDevice/USBSerial/CircBuffer.h
@@ -57,7 +57,7 @@ private:
     volatile uint16_t write;
     volatile uint16_t read;
     static const int size = Size+1;  //a modern optimizer should be able to remove this so it uses no ram.
-    T buf[Size];
+    T buf[Size+1];
 };
 
 #endif