diff --git a/src/core/include/mp_units/bits/unit_concepts.h b/src/core/include/mp_units/bits/unit_concepts.h
index b39b41c4..3c2f1c46 100644
--- a/src/core/include/mp_units/bits/unit_concepts.h
+++ b/src/core/include/mp_units/bits/unit_concepts.h
@@ -24,6 +24,7 @@
 
 #include <mp_units/bits/expression_template.h>
 #include <mp_units/bits/magnitude.h>
+#include <mp_units/bits/quantity_spec_concepts.h>
 #include <mp_units/bits/symbol_text.h>
 
 namespace mp_units {