diff --git a/source/de/anomic/kelondro/blob/ArrayStack.java b/source/de/anomic/kelondro/blob/ArrayStack.java
index 6879c678d..5f036702b 100755
--- a/source/de/anomic/kelondro/blob/ArrayStack.java
+++ b/source/de/anomic/kelondro/blob/ArrayStack.java
@@ -445,7 +445,7 @@ public class ArrayStack implements BLOB {
      * ask for the number of blob entries in each blob of the blob array
      * @return the number of entries in each blob
      */
-    public int[] sizes() {
+    public synchronized int[] sizes() {
         int[] s = new int[blobs.size()];
         int c = 0;
         for (blobItem bi: blobs) s[c++] = bi.blob.size();