diff --git a/src/core/include/mp-units/framework/quantity_spec.h b/src/core/include/mp-units/framework/quantity_spec.h
index 5342eb459..c4497681a 100644
--- a/src/core/include/mp-units/framework/quantity_spec.h
+++ b/src/core/include/mp-units/framework/quantity_spec.h
@@ -37,6 +37,7 @@
 #include <mp-units/framework/reference_concepts.h>
 #include <mp-units/framework/representation_concepts.h>
 #include <mp-units/framework/symbolic_expression.h>
+#include <mp-units/framework/unit_concepts.h>
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
 #ifdef MP_UNITS_IMPORT_STD