diff --git a/libraries/AP_AHRS/AP_AHRS.cpp b/libraries/AP_AHRS/AP_AHRS.cpp
index 4bfb122993..972a26e73d 100644
--- a/libraries/AP_AHRS/AP_AHRS.cpp
+++ b/libraries/AP_AHRS/AP_AHRS.cpp
@@ -587,7 +587,7 @@ void AP_AHRS::update_EKF3(void)
                     _accel_ef_ekf[i] = _dcm_matrix * accel;
                 }
             }
-            _accel_ef_ekf_blended = _accel_ef_ekf[_ins.get_primary_accel()];
+            _accel_ef_ekf_blended = _accel_ef_ekf[primary_imu>=0?primary_imu:_ins.get_primary_accel()];
             nav_filter_status filt_state;
             EKF3.getFilterStatus(-1,filt_state);
             update_notify_from_filter_status(filt_state);