diff --git a/weed/topology/topology.go b/weed/topology/topology.go
index a11a1bac6..e217617e9 100644
--- a/weed/topology/topology.go
+++ b/weed/topology/topology.go
@@ -221,7 +221,7 @@ func (t *Topology) SyncDataNodeRegistration(volumes []*master_pb.VolumeInformati
 	}
 	for _, v := range changedVolumes {
 		vl := t.GetVolumeLayout(v.Collection, v.ReplicaPlacement, v.Ttl)
-		vl.ensureCorrectWritables(&v)
+		vl.EnsureCorrectWritables(&v)
 	}
 	return
 }
diff --git a/weed/topology/volume_layout.go b/weed/topology/volume_layout.go
index e7659e0eb..dddcfc9c9 100644
--- a/weed/topology/volume_layout.go
+++ b/weed/topology/volume_layout.go
@@ -198,6 +198,13 @@ func (vl *VolumeLayout) UnRegisterVolume(v *storage.VolumeInfo, dn *DataNode) {
 	}
 }
 
+func (vl *VolumeLayout) EnsureCorrectWritables(v *storage.VolumeInfo) {
+	vl.accessLock.Lock()
+	defer vl.accessLock.Unlock()
+
+	vl.ensureCorrectWritables(v)
+}
+
 func (vl *VolumeLayout) ensureCorrectWritables(v *storage.VolumeInfo) {
 	if vl.enoughCopies(v.Id) && vl.isWritable(v) {
 		if !vl.oversizedVolumes.IsTrue(v.Id) {
diff --git a/weed/topology/volume_location_list.go b/weed/topology/volume_location_list.go
index 6acd70f2f..64c13ca52 100644
--- a/weed/topology/volume_location_list.go
+++ b/weed/topology/volume_location_list.go
@@ -32,6 +32,9 @@ func (dnll *VolumeLocationList) Head() *DataNode {
 }
 
 func (dnll *VolumeLocationList) Length() int {
+	if dnll == nil {
+		return 0
+	}
 	return len(dnll.list)
 }