diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java
index 8653c3228..d42d348a1 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java
@@ -2371,10 +2371,14 @@ public boolean prepareBitmapIndex(ProgressMonitor pm) throws IOException {
 
 		int numCommits = objectsLists[OBJ_COMMIT].size();
 		List<ObjectToPack> byName = sortByName();
+		// Reset sortedByName before the array that it points to is mutated by
+		// PackBitmapIndexBuilder, to prevent other methods referencing the
+		// mutated array afterwards.
 		sortedByName = null;
 		objectsLists = null;
 		objectsMap = null;
 		writeBitmaps = new PackBitmapIndexBuilder(byName);
+		// Allow byName to be GC'd if JVM GC runs before the end of the method.
 		byName = null;
 
 		PackWriterBitmapPreparer bitmapPreparer = new PackWriterBitmapPreparer(