diff --git a/src/core/include/mp-units/framework/quantity_spec.h b/src/core/include/mp-units/framework/quantity_spec.h
index 34ea2c0e..80a6d0ad 100644
--- a/src/core/include/mp-units/framework/quantity_spec.h
+++ b/src/core/include/mp-units/framework/quantity_spec.h
@@ -26,6 +26,7 @@
 #include <mp-units/bits/hacks.h>
 #include <mp-units/bits/module_macros.h>
 #include <mp-units/bits/quantity_spec_hierarchy.h>
+#include <mp-units/bits/ratio.h>
 #include <mp-units/compat_macros.h>
 #include <mp-units/ext/algorithm.h>
 #include <mp-units/ext/type_name.h>