diff --git a/src/systems/isq_angle/include/mp_units/systems/isq_angle/isq_angle.h b/src/systems/isq_angle/include/mp_units/systems/isq_angle/isq_angle.h
index 8f87063c..0f325c3f 100644
--- a/src/systems/isq_angle/include/mp_units/systems/isq_angle/isq_angle.h
+++ b/src/systems/isq_angle/include/mp_units/systems/isq_angle/isq_angle.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include <mp_units/quantity_spec.h>
+#include <mp_units/reference.h>
 #include <mp_units/systems/angular/angular.h>
 #include <mp_units/systems/isq/isq.h>