diff --git a/lib/blocklib.php b/lib/blocklib.php
index d5d1c3b137218..508b94c1e1479 100644
--- a/lib/blocklib.php
+++ b/lib/blocklib.php
@@ -1467,8 +1467,10 @@ public function process_url_move() {
         } else {
             $newweight = ceil($newweight);
             for ($weight = $bestgap - 1; $weight >= $newweight; $weight--) {
-                foreach ($usedweights[$weight] as $biid) {
-                    $this->reposition_block($biid, $newregion, $weight + 1);
+                if (array_key_exists($weight, $usedweights)) {
+                    foreach ($usedweights[$weight] as $biid) {
+                        $this->reposition_block($biid, $newregion, $weight + 1);
+                    }
                 }
             }
             $this->reposition_block($block->instance->id, $newregion, $newweight);