diff --git a/tests/io_mock.h b/tests/io_mock.h
index 873477470..f8f75adcd 100644
--- a/tests/io_mock.h
+++ b/tests/io_mock.h
@@ -45,6 +45,7 @@
 
 /* Define struct pci_dev to avoid dependency on pci.h */
 struct pci_dev {
+	char padding[18];
 	unsigned int device_id;
 };