diff --git a/src/core/include/mp-units/bits/external/fixed_string.h b/src/core/include/mp-units/bits/external/fixed_string.h
index 402140c1..ad8d302f 100644
--- a/src/core/include/mp-units/bits/external/fixed_string.h
+++ b/src/core/include/mp-units/bits/external/fixed_string.h
@@ -34,6 +34,17 @@
 #include <cstdlib>
 #include <ostream>
 #include <string_view>
+
+#if MP_UNITS_USE_FMTLIB
+MP_UNITS_DIAGNOSTIC_PUSH
+MP_UNITS_DIAGNOSTIC_IGNORE_UNREACHABLE
+MP_UNITS_DIAGNOSTIC_IGNORE_SHADOW
+#include <fmt/format.h>
+MP_UNITS_DIAGNOSTIC_POP
+#else
+#include <format>
+#endif
+
 #endif
 
 MP_UNITS_EXPORT