diff --git a/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.cpp b/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.cpp
index ce856bb5e3..fea86b8afa 100644
--- a/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.cpp
+++ b/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.cpp
@@ -309,11 +309,14 @@ void AP_OpticalFlow_Pixart::timer(void)
     }
     
 #if 0
+    static uint32_t last_print_ms;
     static int fd = -1;
     if (fd == -1) {
         fd = open("/dev/ttyACM0", O_WRONLY);
     }
     // used for debugging
+    static int32_t sum_x;
+    static int32_t sum_y;
     sum_x += burst.delta_x;
     sum_y += burst.delta_y;
     
diff --git a/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h b/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h
index abc2c99ce6..9c5e11d09b 100644
--- a/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h
+++ b/libraries/AP_OpticalFlow/AP_OpticalFlow_Pixart.h
@@ -72,9 +72,6 @@ private:
     void timer(void);
     void motion_burst(void);
 
-    int32_t sum_x;
-    int32_t sum_y;
-    uint32_t last_print_ms;
     uint32_t last_burst_us;
     uint32_t last_update_ms;
 };