diff --git a/src/org/ntlab/deltaViewer/CollaborationViewer.java b/src/org/ntlab/deltaViewer/CollaborationViewer.java
index 830674d..1db4730 100644
--- a/src/org/ntlab/deltaViewer/CollaborationViewer.java
+++ b/src/org/ntlab/deltaViewer/CollaborationViewer.java
@@ -386,6 +386,9 @@
 					}
 					if (!objectToVertexMap.containsKey(ref.getDstObjectId())) {
 						objectToVertexMap.put(ref.getDstObjectId(), new ObjectVertex(ref.getDstClassName(), null, 0, 0));
+					} else {
+						mxgraph.removeCells(new Object[] {objectToVertexMap.get(ref.getDstObjectId()).getCell()});
+						objectToVertexMap.get(ref.getDstObjectId()).setCell(null);
 					}
 				}
 			}
diff --git a/src/org/ntlab/deltaViewer/MagnetRONViewer.java b/src/org/ntlab/deltaViewer/MagnetRONViewer.java
index 7af37f3..6fba4dc 100644
--- a/src/org/ntlab/deltaViewer/MagnetRONViewer.java
+++ b/src/org/ntlab/deltaViewer/MagnetRONViewer.java
@@ -808,8 +808,8 @@
 			mxCell targetVertexCell = null;
 			
 			if (methodExec != null) {
-			sourceVertexCell = (mxICell)methodExecToVertexMap.get(methodExec).getCell();
-			targetVertexCell = (mxCell)methodExecToVertexMap.get(calledMethodExec).getCell();
+				sourceVertexCell = (mxICell)methodExecToVertexMap.get(methodExec).getCell();
+				targetVertexCell = (mxCell)methodExecToVertexMap.get(calledMethodExec).getCell();
 			}
 //			if(!calledMethodExec.isStatic()) {
 			// Add a vertex to the graph in a transactional fashion. The vertex is actually a 'cell' in jgraphx terminology.