diff --git a/tools/libs/light/libxl_virtio.c b/tools/libs/light/libxl_virtio.c
index 6a38def2fa..faada49e18 100644
--- a/tools/libs/light/libxl_virtio.c
+++ b/tools/libs/light/libxl_virtio.c
@@ -45,12 +45,12 @@ static int libxl__set_xenstore_virtio(libxl__gc *gc, uint32_t domid,
     const char *transport = libxl_virtio_transport_to_string(virtio->transport);
 
     flexarray_append_pair(back, "irq", GCSPRINTF("%u", virtio->irq));
-    flexarray_append_pair(back, "base", GCSPRINTF("%lu", virtio->base));
+    flexarray_append_pair(back, "base", GCSPRINTF("%#"PRIx64, virtio->base));
     flexarray_append_pair(back, "type", GCSPRINTF("%s", virtio->type));
     flexarray_append_pair(back, "transport", GCSPRINTF("%s", transport));
 
     flexarray_append_pair(front, "irq", GCSPRINTF("%u", virtio->irq));
-    flexarray_append_pair(front, "base", GCSPRINTF("%lu", virtio->base));
+    flexarray_append_pair(front, "base", GCSPRINTF("%#"PRIx64, virtio->base));
     flexarray_append_pair(front, "type", GCSPRINTF("%s", virtio->type));
     flexarray_append_pair(front, "transport", GCSPRINTF("%s", transport));