diff --git a/Adafruit_SSD1306.h b/Adafruit_SSD1306.h
index 939725b..18b4d4f 100644
--- a/Adafruit_SSD1306.h
+++ b/Adafruit_SSD1306.h
@@ -48,7 +48,7 @@
   typedef volatile RwReg    PortReg;
   typedef uint32_t          PortMask;
   #define HAVE_PORTREG
-#elif defined(__arm__) || defined(ARDUINO_FEATHER52)
+#elif (defined(__arm__) || defined(ARDUINO_FEATHER52)) && !defined(ARDUINO_ARCH_MBED)
   typedef volatile uint32_t PortReg;
   typedef uint32_t          PortMask;
   #define HAVE_PORTREG