--- a/graal/com.oracle.truffle.api/src/com/oracle/truffle/api/nodes/Node.java	Sun Oct 20 03:26:03 2013 +0200
+++ b/graal/com.oracle.truffle.api/src/com/oracle/truffle/api/nodes/Node.java	Sun Oct 20 03:50:51 2013 +0200
@@ -171,6 +171,7 @@
      * @return the new node
      */
     public final <T extends Node> T replace(T newNode, String reason) {
+        CompilerDirectives.transferToInterpreter();
         if (this.getParent() == null) {
             throw new IllegalStateException("This node cannot be replaced, because it does not yet have a parent.");
         }
@@ -196,18 +197,23 @@
         if (rootNode == null) {
             throw new UnsupportedOperationException("Tree does not have a root node.");
         }
-        rootNode.fixupChildren();
+        int fixCount = rootNode.fixupChildren();
+        assert fixCount != 0 : "sanity check failed: missing @Child[ren] or adoptChild?";
+        // if nothing had to be fixed, rewrite failed due to node not being a proper child.
     }
 
-    private void fixupChildren() {
+    private int fixupChildren() {
+        int fixCount = 0;
         for (Node child : getChildren()) {
             if (child != null) {
                 if (child.parent != this) {
                     child.parent = this;
+                    fixCount++;
                 }
-                child.fixupChildren();
+                fixCount += child.fixupChildren();
             }
         }
+        return fixCount;
     }
 
     /**