diff --git a/src/core-fmt/include/mp-units/bits/fmt.h b/src/core-fmt/include/mp-units/bits/fmt.h
index 559c720c..95ef4549 100644
--- a/src/core-fmt/include/mp-units/bits/fmt.h
+++ b/src/core-fmt/include/mp-units/bits/fmt.h
@@ -27,6 +27,8 @@
 //
 // For the license information refer to format.h.
 
+#pragma once
+
 #include <gsl/gsl-lite.hpp>
 #include <mp-units/bits/fmt_hacks.h>
 #include <concepts>