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 f1a0fa3a..1ad0a5b4 100644
--- a/src/core/include/mp-units/bits/external/fixed_string.h
+++ b/src/core/include/mp-units/bits/external/fixed_string.h
@@ -29,7 +29,7 @@
 // IWYU pragma: begin_exports
 #include <compare>
 #include <cstdlib>
-#include <iostream>
+#include <ostream>
 // IWYU pragma: end_exports
 
 #include <cstddef>