diff --git a/lib/blocklib.php b/lib/blocklib.php
index 5d1fb248c74..a44c18567ea 100644
--- a/lib/blocklib.php
+++ b/lib/blocklib.php
@@ -1599,8 +1599,10 @@ class block_manager {
         if ($bestgap < $newweight) {
             $newweight = floor($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);