diff --git a/libraries/AP_OpticalFlow/OpticalFlow_backend.cpp b/libraries/AP_OpticalFlow/OpticalFlow_backend.cpp
index af167953de..2341cf7f4f 100644
--- a/libraries/AP_OpticalFlow/OpticalFlow_backend.cpp
+++ b/libraries/AP_OpticalFlow/OpticalFlow_backend.cpp
@@ -17,6 +17,12 @@
 
 extern const AP_HAL::HAL& hal;
 
+OpticalFlow_backend::OpticalFlow_backend(OpticalFlow &_frontend) :
+    frontend(_frontend)
+{
+    _sem = hal.util->new_semaphore();    
+}
+
 // update the frontend
 void OpticalFlow_backend::_update_frontend(const struct OpticalFlow::OpticalFlow_state &state)
 {
diff --git a/libraries/AP_OpticalFlow/OpticalFlow_backend.h b/libraries/AP_OpticalFlow/OpticalFlow_backend.h
index 5c821168b3..e0e4226f11 100644
--- a/libraries/AP_OpticalFlow/OpticalFlow_backend.h
+++ b/libraries/AP_OpticalFlow/OpticalFlow_backend.h
@@ -20,17 +20,13 @@
 
 #include "OpticalFlow.h"
 
-extern const AP_HAL::HAL& hal;
-
 class OpticalFlow_backend
 {
     friend class OpticalFlow;
 
 public:
     // constructor
-    OpticalFlow_backend(OpticalFlow &_frontend) : frontend(_frontend) {
-        _sem = hal.util->new_semaphore();    
-    }
+    OpticalFlow_backend(OpticalFlow &_frontend);
 
     // init - initialise sensor
     virtual void init() = 0;