diff --git a/src/core/include/mp-units/ext/fixed_string.h b/src/core/include/mp-units/ext/fixed_string.h
index 12987ba2..ca4d8284 100644
--- a/src/core/include/mp-units/ext/fixed_string.h
+++ b/src/core/include/mp-units/ext/fixed_string.h
@@ -34,6 +34,7 @@
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
 #include <mp-units/ext/contracts.h>
 #include <compare>  // IWYU pragma: export
+#include <concepts>
 #include <cstddef>
 #include <cstdlib>
 #include <ranges>