diff --git a/components/usb/test/common/test_usb_mock_classes.h b/components/usb/test/common/test_usb_mock_classes.h
index b225a37517..f057670e46 100644
--- a/components/usb/test/common/test_usb_mock_classes.h
+++ b/components/usb/test/common/test_usb_mock_classes.h
@@ -21,7 +21,7 @@ extern "C" {
 
 // ---------------------------------------------------- MSC SCSI -------------------------------------------------------
 
-const char *MSC_CLIENT_TAG;
+extern const char *MSC_CLIENT_TAG;
 
 /*
 Note: The mock MSC SCSI tests requires that USB flash drive be connected. The flash drive should...