diff --git a/ext/-test-/thread/instrumentation/instrumentation.c b/ext/-test-/thread/instrumentation/instrumentation.c
index 2bbce1179a..d2a2c2740b 100644
--- a/ext/-test-/thread/instrumentation/instrumentation.c
+++ b/ext/-test-/thread/instrumentation/instrumentation.c
@@ -21,7 +21,7 @@ static RB_THREAD_LOCAL_SPECIFIER unsigned int local_ready_count = 0;
 static RB_THREAD_LOCAL_SPECIFIER unsigned int local_resumed_count = 0;
 static RB_THREAD_LOCAL_SPECIFIER unsigned int local_suspended_count = 0;
 
-void
+static void
 ex_callback(rb_event_flag_t event, const rb_internal_thread_event_data_t *event_data, void *user_data)
 {
     switch (event) {