diff --git a/libraries/AP_InertialSensor/AP_InertialSensor.cpp b/libraries/AP_InertialSensor/AP_InertialSensor.cpp
index 3f10354b84..2c04e511e1 100644
--- a/libraries/AP_InertialSensor/AP_InertialSensor.cpp
+++ b/libraries/AP_InertialSensor/AP_InertialSensor.cpp
@@ -940,7 +940,8 @@ AP_InertialSensor::detect_backends(void)
 
 #if HAL_EXTERNAL_AHRS_ENABLED
     // if enabled, make the first IMU the external AHRS
-    if (int8_t serial_port = AP::externalAHRS().get_port() >= 0) {
+    const int8_t serial_port = AP::externalAHRS().get_port();
+    if (serial_port >= 0) {
         ADD_BACKEND(new AP_InertialSensor_ExternalAHRS(*this, serial_port));
     }
 #endif