diff --git a/src/core/include/mp-units/bits/ratio.h b/src/core/include/mp-units/bits/ratio.h
index 89c4a52c..529d4d17 100644
--- a/src/core/include/mp-units/bits/ratio.h
+++ b/src/core/include/mp-units/bits/ratio.h
@@ -23,6 +23,7 @@
 #pragma once
 
 #include <gsl/gsl-lite.hpp>
+#include <mp-units/bits/external/hacks.h>
 #include <compare>
 #include <cstdint>
 #include <numeric>