diff --git a/IO_CMRI.cpp b/IO_CMRI.cpp
index 992b00b..1dfa2fc 100644
--- a/IO_CMRI.cpp
+++ b/IO_CMRI.cpp
@@ -40,7 +40,7 @@ CMRIbus::CMRIbus(uint8_t busNo, HardwareSerial &serial, unsigned long baud, uint
   // Each byte is one start bit, 8 data bits and 1 or 2 stop bits, assume 11 bits per byte.  
   // Calculate timeout based on treble this time.
   _timeoutPeriod = 3 * 11 * 262 * 1000UL / (_baud / 1000UL);
-#if ARDUINOCMRI_COMPATIBLE
+#if defined(ARDUINOCMRI_COMPATIBLE)
   // NOTE: The ArduinoCMRI library, unless modified, contains a 'delay(50)' between 
   // receiving the end of the prompt message and starting to send the response.  This
   // is allowed for below.
@@ -240,7 +240,7 @@ void CMRIbus::enableTransmitter() {
   // it's preceded by an odd number of SYN characters.
   // So send a SYN followed by a NUL in that case.
   _serial->write(SYN);
-#if ARDUINOCMRI_COMPATIBLE
+#if defined(ARDUINOCMRI_COMPATIBLE)
   _serial->write(NUL);  // Reset the ArduinoCMRI library's parser
 #endif
 }
diff --git a/IO_CMRI.h b/IO_CMRI.h
index 4f2d9b9..bb1abfb 100644
--- a/IO_CMRI.h
+++ b/IO_CMRI.h
@@ -36,7 +36,7 @@
  * Various work-arounds may be enabled within the driver by adding the following line to your config.h file,
  * to allow nodes running the ArduinoCMRI library to communicate:
  * 
- *  #define ARDUINOCMRI_COMPATIBLE TRUE
+ *  #define ARDUINOCMRI_COMPATIBLE
  * 
  * CMRINode
  * ========
@@ -222,7 +222,7 @@ public:
   // Device-specific initialisation
   void _begin() override {
     // CMRInet spec states one stop bit, JMRI and ArduinoCMRI use two stop bits
-#if ARDUINOCMRI_COMPATIBLE
+#if defined(ARDUINOCMRI_COMPATIBLE)
     _serial->begin(_baud, SERIAL_8N2);
 #else
     _serial->begin(_baud, SERIAL_8N1);