diff --git a/src/device/usbd.c b/src/device/usbd.c
index 661ba95a3..83217a869 100644
--- a/src/device/usbd.c
+++ b/src/device/usbd.c
@@ -1098,7 +1098,7 @@ static bool process_get_descriptor(uint8_t rhport, tusb_control_request_t const
   }
 }
 
-bool process_test_mode_cb(uint8_t rhport, uint8_t stage, tusb_control_request_t const * request)
+static bool process_test_mode_cb(uint8_t rhport, uint8_t stage, tusb_control_request_t const * request)
 {
   // At this point it should already be ensured that dcd_enter_test_mode() is defined