diff --git a/io.hpp b/io.hpp
index ee727c91..dedfc514 100644
--- a/io.hpp
+++ b/io.hpp
@@ -220,7 +220,7 @@ namespace cruft {
 
     class stream_error : public std::exception {
     public:
-        virtual const char* what (void) const noexcept
+        const char* what (void) const noexcept override
         { return "error on C stream"; }
     };
 }