diff --git a/weed/topology/node.go b/weed/topology/node.go
index 206a9aff4..5dac4966b 100644
--- a/weed/topology/node.go
+++ b/weed/topology/node.go
@@ -168,7 +168,7 @@ func (n *NodeImpl) ReserveOneVolume(r int) (assignedNode *DataNode, err error) {
 			}
 		}
 	}
-	return
+	return nil, errors.New("No free volume slot found!")
 }
 
 func (n *NodeImpl) UpAdjustMaxVolumeCountDelta(maxVolumeCountDelta int) { //can be negative