diff --git a/src/string_func.h b/src/string_func.h
index 4fe9d7b825..787fd6413e 100644
--- a/src/string_func.h
+++ b/src/string_func.h
@@ -40,7 +40,7 @@ int CDECL vseprintf(char *str, const char *last, const char *format, va_list ap)
 char *CDECL str_fmt(const char *str, ...) WARN_FORMAT(1, 2);
 
 void str_validate(char *str, const char *last, StringValidationSettings settings = SVS_REPLACE_WITH_QUESTION_MARK) NOACCESS(2);
-std::string str_validate(const std::string &str, StringValidationSettings settings = SVS_REPLACE_WITH_QUESTION_MARK);
+[[nodiscard]] std::string str_validate(const std::string &str, StringValidationSettings settings = SVS_REPLACE_WITH_QUESTION_MARK);
 void ValidateString(const char *str);
 
 void str_fix_scc_encoded(char *str, const char *last) NOACCESS(2);