diff --git a/libraries/AP_OpticalFlow/AP_OpticalFlow_SITL.cpp b/libraries/AP_OpticalFlow/AP_OpticalFlow_SITL.cpp
index d6208a6aae..7ee6662b7f 100644
--- a/libraries/AP_OpticalFlow/AP_OpticalFlow_SITL.cpp
+++ b/libraries/AP_OpticalFlow/AP_OpticalFlow_SITL.cpp
@@ -114,7 +114,7 @@ void AP_OpticalFlow_SITL::update(void)
         // cope with updates to the delay control
         if (_sitl->flow_delay > 0 &&
             (uint8_t)(_sitl->flow_delay) > ARRAY_SIZE(optflow_data)) {
-            _sitl->flow_delay = ARRAY_SIZE(optflow_data);
+            _sitl->flow_delay.set(ARRAY_SIZE(optflow_data));
         }
         optflow_delay = _sitl->flow_delay;
         for (uint8_t i=0; i<optflow_delay; i++) {