Better way to eliminate FindBugs warning?

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@1073116 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/fop/layoutmgr/BlockLayoutManager.java

@@ -141,6 +141,7 @@ public class BlockLayoutManager extends BlockStackingLayoutManager
             return childLM.getNextKnuthElements(childLC, alignment);
         } else {
             if (childLM instanceof LineLayoutManager) {
+                assert (restartPosition instanceof LeafPosition);
                 return ((LineLayoutManager) childLM).getNextKnuthElements(childLC, alignment,
                         (LeafPosition) restartPosition);
             } else {