diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java b/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java
index 6b4ed80e1..476592f5d 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/revplot/PlotWalk.java
@@ -53,12 +53,13 @@
 
 import org.eclipse.jgit.JGitText;
 import org.eclipse.jgit.lib.AnyObjectId;
-import org.eclipse.jgit.lib.Commit;
+import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.lib.Ref;
 import org.eclipse.jgit.lib.Repository;
-import org.eclipse.jgit.lib.Tag;
 import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevObject;
 import org.eclipse.jgit.revwalk.RevSort;
+import org.eclipse.jgit.revwalk.RevTag;
 import org.eclipse.jgit.revwalk.RevWalk;
 
 /** Specialized RevWalk for visualization of a commit graph. */
@@ -115,8 +116,8 @@ protected Ref[] getTags(final AnyObjectId commitId) {
 	class PlotRefComparator implements Comparator<Ref> {
 		public int compare(Ref o1, Ref o2) {
 			try {
-				Object obj1 = getRepository().mapObject(o1.getObjectId(), o1.getName());
-				Object obj2 = getRepository().mapObject(o2.getObjectId(), o2.getName());
+				RevObject obj1 = parseAny(o1.getObjectId());
+				RevObject obj2 = parseAny(o2.getObjectId());
 				long t1 = timeof(obj1);
 				long t2 = timeof(obj2);
 				if (t1 > t2)
@@ -129,11 +130,15 @@ public int compare(Ref o1, Ref o2) {
 				return 0;
 			}
 		}
-		long timeof(Object o) {
-			if (o instanceof Commit)
-				return ((Commit)o).getCommitter().getWhen().getTime();
-			if (o instanceof Tag)
-				return ((Tag)o).getTagger().getWhen().getTime();
+
+		long timeof(RevObject o) {
+			if (o instanceof RevCommit)
+				return ((RevCommit) o).getCommitTime();
+			if (o instanceof RevTag) {
+				RevTag tag = (RevTag) o;
+				PersonIdent who = tag.getTaggerIdent();
+				return who != null ? who.getWhen().getTime() : 0;
+			}
 			return 0;
 		}
 	}