diff --git a/src/org/ntlab/deltaViewer/CollaborationLayout.java b/src/org/ntlab/deltaViewer/CollaborationLayout.java
index 30f331b..edadf05 100644
--- a/src/org/ntlab/deltaViewer/CollaborationLayout.java
+++ b/src/org/ntlab/deltaViewer/CollaborationLayout.java
@@ -114,7 +114,12 @@
 					String dstObjId = methodExec.getReturnValue().getId();
 					String srcClassName = methodExec.getThisClassName();
 					String dstClassName = methodExec.getReturnValue().getActualType();
-					idx = references.indexOf(new Reference(srcObjId, dstObjId, srcClassName, dstClassName));
+					Reference r = new Reference(srcObjId, dstObjId, srcClassName, dstClassName);
+					idx = references.indexOf(r);
+					if (idx < 0) {
+						references.add(r);
+						idx = references.indexOf(r);
+					}					
 				}
 			} else if (a.getAliasType() == AliasType.CONSTRACTOR_INVOCATION) {
 				MethodInvocation c = (MethodInvocation) a.getOccurrencePoint().getStatement();