diff --git a/apps/vtn/vtnmgr/src/main/java/org/onosproject/vtn/manager/impl/VtnManager.java b/apps/vtn/vtnmgr/src/main/java/org/onosproject/vtn/manager/impl/VtnManager.java
index b1e2f94..c3c5af9 100644
--- a/apps/vtn/vtnmgr/src/main/java/org/onosproject/vtn/manager/impl/VtnManager.java
+++ b/apps/vtn/vtnmgr/src/main/java/org/onosproject/vtn/manager/impl/VtnManager.java
@@ -1553,6 +1553,7 @@
             exVmPort = exVmPortList.iterator().next();
         }
         if (exVmPort == null) {
+            log.debug("exVM Port is null {}.", floatingIp);
             return null;
         }
         Set<FixedIp> fixedIps = exVmPort.fixedIps();
@@ -1565,6 +1566,7 @@
             }
         }
         if (subnetId == null) {
+            log.debug("subnetId is null {}.", floatingIp);
             return null;
         }
         Subnet subnet = subnetService.getSubnet(subnetId);
@@ -1587,6 +1589,7 @@
             }
         }
         if (host == null) {
+            log.debug("Host is null {}.", floatingIp);
             return null;
         } else {
             return host.location().deviceId();