diff --git a/src/org/ntlab/deltaViewer/DeltaViewer.java b/src/org/ntlab/deltaViewer/DeltaViewer.java
index 4f52ab5..ea36f70 100644
--- a/src/org/ntlab/deltaViewer/DeltaViewer.java
+++ b/src/org/ntlab/deltaViewer/DeltaViewer.java
@@ -1301,11 +1301,11 @@
 
 		if (curFrame == 0) {
 			MethodExecution callerMethodExec = methodExec.getCallerMethodExecution();
-			if (methodExecSignature != callerMethodExec.getSignature() && objectToVertexMap.containsKey(callerMethodExec.getThisObjId())) {
+			if (callerMethodExec != null && methodExecSignature != callerMethodExec.getSignature() && objectToVertexMap.containsKey(callerMethodExec.getThisObjId()) && eStructure.getCoordinator() == callerMethodExec) {
 				createMethodExecutionVertex(callerMethodExec.getThisObjId(), callerMethodExec.getSignature(), callerMethodExec);
 			}
 			createMethodExecutionVertex(objId, methodExecSignature, methodExec);
-		} else if (alias.getObjectId().matches("0")) {
+		} else if (alias.getObjectId().matches("0") && !methodExecToVertexMap.containsKey(methodExec)) {
 			createMethodExecutionVertex(objId, methodExecSignature, methodExec);
 		}
 	}