diff --git a/hw/pci-bridge/pci_expander_bridge.c b/hw/pci-bridge/pci_expander_bridge.c
index 11623a5666..40ffbc4e08 100644
--- a/hw/pci-bridge/pci_expander_bridge.c
+++ b/hw/pci-bridge/pci_expander_bridge.c
@@ -85,8 +85,14 @@ static uint16_t pxb_bus_numa_node(PCIBus *bus)
 static void prop_pxb_uid_get(Object *obj, Visitor *v, const char *name,
                              void *opaque, Error **errp)
 {
-    uint32_t uid = pci_bus_num(PCI_BUS(obj));
+    PCIBus *bus = PCI_BUS(obj);
+    uint32_t uid;
 
+    if (!bus->parent_dev) {
+        error_setg(errp, "bus not attached to a device");
+        return;
+    }
+    uid = pci_bus_num(bus);
     visit_type_uint32(v, name, &uid, errp);
 }