diff --git a/monitor.c b/monitor.c
index fbb678d..260cc02 100644
--- a/monitor.c
+++ b/monitor.c
@@ -549,7 +549,11 @@
 {
     const char *tp_name = qdict_get_str(qdict, "name");
     bool new_state = qdict_get_bool(qdict, "option");
-    st_change_trace_event_state(tp_name, new_state);
+    int ret = st_change_trace_event_state(tp_name, new_state);
+
+    if (!ret) {
+        monitor_printf(mon, "unknown event name \"%s\"\n", tp_name);
+    }
 }
 
 static void do_trace_file(Monitor *mon, const QDict *qdict)
diff --git a/simpletrace.c b/simpletrace.c
index f849e42..deb1e07 100644
--- a/simpletrace.c
+++ b/simpletrace.c
@@ -246,12 +246,14 @@
     return NULL; /* indicates end of list reached without a match */
 }
 
-void st_change_trace_event_state(const char *tname, bool tstate)
+bool st_change_trace_event_state(const char *tname, bool tstate)
 {
     TraceEvent *tp;
 
     tp = find_trace_event_by_name(tname);
     if (tp) {
         tp->state = tstate;
+        return true;
     }
+    return false;
 }
diff --git a/simpletrace.h b/simpletrace.h
index cf35897..72614ec 100644
--- a/simpletrace.h
+++ b/simpletrace.h
@@ -31,7 +31,7 @@
 void trace6(TraceEventID event, uint64_t x1, uint64_t x2, uint64_t x3, uint64_t x4, uint64_t x5, uint64_t x6);
 void st_print_trace(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...));
 void st_print_trace_events(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...));
-void st_change_trace_event_state(const char *tname, bool tstate);
+bool st_change_trace_event_state(const char *tname, bool tstate);
 void st_print_trace_file_status(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...));
 void st_set_trace_file_enabled(bool enable);
 bool st_set_trace_file(const char *file);