diff --git a/test/mbox.c b/test/mbox.c
index 8897924..a351250 100644
--- a/test/mbox.c
+++ b/test/mbox.c
@@ -290,6 +290,8 @@
 	struct mbox_test_context *arg;
 	void *map;
 
+	assert(test.flash.fd > 2);
+
 	/* Sanity check */
 	arg = container_of(context, struct mbox_test_context, context);
 	assert(&test == arg);