diff --git a/modules/netbox/catalogue.tf b/modules/netbox/catalogue.tf
index 9b01269..bb6c21f 100644
--- a/modules/netbox/catalogue.tf
+++ b/modules/netbox/catalogue.tf
@@ -71,4 +71,7 @@ resource "netbox_device" "network" {
   serial         = each.value.serial
   asset_tag      = each.value.asset_tag
   custom_fields  = each.value.chassis_mac != null ? { chassis_mac = lower(each.value.chassis_mac) } : null
+
+  # chassis_mac must exist as a field before a device can carry a value for it.
+  depends_on = [netbox_custom_field.this]
 }