diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index 13b1fe694b90..bd16b17ba4d6 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -375,11 +375,11 @@ static int ubd_setup_common(char *str, int *index_out, char **error_out)
 		file = NULL;
 
 	backing_file = strsep(&str, ",:");
-	if (*backing_file == '\0')
+	if (backing_file && *backing_file == '\0')
 		backing_file = NULL;
 
 	serial = strsep(&str, ",:");
-	if (*serial == '\0')
+	if (serial && *serial == '\0')
 		serial = NULL;
 
 	if (backing_file && ubd_dev->no_cow) {