diff --git a/ArduCopter/ArduCopter.pde b/ArduCopter/ArduCopter.pde
index 1eaf0cf16c..43c5118071 100644
--- a/ArduCopter/ArduCopter.pde
+++ b/ArduCopter/ArduCopter.pde
@@ -68,6 +68,7 @@
 #include <I2C.h>                        // Arduino I2C lib
 #include <SPI.h>                        // Arduino SPI lib
 #include <SPI3.h>               // SPI3 library
+#include <AP_Semaphore.h>   // for removing conflict between optical flow and dataflash on SPI3 bus
 #include <DataFlash.h>      // ArduPilot Mega Flash Memory Library
 #include <AP_ADC.h>         // ArduPilot Mega Analog to Digital Converter Library
 #include <AP_AnalogSource.h>