diff --git a/libraries/USBDevice/USBSerial/CircBuffer.h b/libraries/USBDevice/USBSerial/CircBuffer.h
index b784a72019..f581f9d2df 100644
--- a/libraries/USBDevice/USBSerial/CircBuffer.h
+++ b/libraries/USBDevice/USBSerial/CircBuffer.h
@@ -22,6 +22,7 @@
 template <class T, int Size>
 class CircBuffer {
 public:
+    CircBuffer():write(0), read(0){}
     bool isFull() {
         return ((write + 1) % size == read);
     };
@@ -53,9 +54,9 @@ public:
     };
 
 private:
-    volatile uint16_t write{0};				//in case older compilers are targeted this should be done in a constructor
-    volatile uint16_t read{0};
-    static constexpr int size{Size+1};
+    volatile uint16_t write;
+    volatile uint16_t read;
+    static const int size = Size+1;
     T buf[Size];
 };