diff --git a/EKF/estimator_interface.h b/EKF/estimator_interface.h
index 8b3e23f317..4e72c4bcb5 100644
--- a/EKF/estimator_interface.h
+++ b/EKF/estimator_interface.h
@@ -180,6 +180,7 @@ public:
 	void setRangeData(uint64_t time_usec, float data);
 
 	// set optical flow data
+	// if optical flow sensor gyro delta angles are not available, set gyroXYZ vector fields to NaN and the EKF will use its internal delta angle data instead
 	void setOpticalFlowData(uint64_t time_usec, flow_message *flow);
 
 	// set external vision position and attitude data