diff --git a/kernel/log.cc b/kernel/log.cc
index cb4648f43..8f9ee2a13 100644
--- a/kernel/log.cc
+++ b/kernel/log.cc
@@ -402,16 +402,11 @@ void log_file_error(const string &filename, int lineno,
 	logv_file_error(filename, lineno, format, ap);
 }
 
-void log_experimental(const char *format, ...)
+void log_experimental(const std::string &str)
 {
-	va_list ap;
-	va_start(ap, format);
-	string s = vstringf(format, ap);
-	va_end(ap);
-
-	if (log_experimentals_ignored.count(s) == 0 && log_experimentals.count(s) == 0) {
-		log_warning("Feature '%s' is experimental.\n", s.c_str());
-		log_experimentals.insert(s);
+	if (log_experimentals_ignored.count(str) == 0 && log_experimentals.count(str) == 0) {
+		log_warning("Feature '%s' is experimental.\n", str);
+		log_experimentals.insert(str);
 	}
 }
 
diff --git a/kernel/log.h b/kernel/log.h
index 1ca60f7a5..ba2c1d9dc 100644
--- a/kernel/log.h
+++ b/kernel/log.h
@@ -121,9 +121,6 @@ extern int log_debug_suppressed;
 
 [[noreturn]] void logv_file_error(const string &filename, int lineno, const char *format, va_list ap);
 
-
-void log_experimental(const char *format, ...) YS_ATTRIBUTE(format(printf, 1, 2));
-
 void set_verific_logging(void (*cb)(int msg_type, const char *message_id, const char* file_path, unsigned int left_line, unsigned int left_col, unsigned int right_line, unsigned int right_col, const char *msg));
 extern void (*log_verific_callback)(int msg_type, const char *message_id, const char* file_path, unsigned int left_line, unsigned int left_col, unsigned int right_line, unsigned int right_col, const char *msg);
 
@@ -170,6 +167,8 @@ inline void log_warning_noprefix(FmtString<TypeIdentity<Args>...> fmt, Args... a
 	log_formatted_warning("", fmt.format(args...));
 }
 
+void log_experimental(const std::string &str);
+
 static inline void log_suppressed() {
 	if (log_debug_suppressed && !log_make_debug) {
 		log("<suppressed ~%d debug messages>\n", log_debug_suppressed);
diff --git a/kernel/register.cc b/kernel/register.cc
index af1823b5b..38b8c8dc8 100644
--- a/kernel/register.cc
+++ b/kernel/register.cc
@@ -253,7 +253,7 @@ void Pass::call(RTLIL::Design *design, std::vector<std::string> args)
 		log_cmd_error("No such command: %s (type 'help' for a command overview)\n", args[0].c_str());
 
 	if (pass_register[args[0]]->experimental_flag)
-		log_experimental("%s", args[0].c_str());
+		log_experimental(args[0]);
 
 	size_t orig_sel_stack_pos = design->selection_stack.size();
 	auto state = pass_register[args[0]]->pre_execute();