check each slice if it's outside, not only first one

src/Tokenizer/Tokens.php

@@ -858,13 +858,6 @@ public function insertSlices(array $slices): void
         $this->setSize($oldSize + $itemsCount);
 
         krsort($slices);
-
-        $firstIndex = array_key_first($slices);
-
-        if ($firstIndex > $oldSize) {
-            throw new \OutOfBoundsException('Cannot insert outside of collection.');
-        }
-
         $insertBound = $oldSize - 1;
 
         // since we only move already existing items around, we directly call into SplFixedArray::offset* methods.
@@ -874,6 +867,10 @@ public function insertSlices(array $slices): void
                 throw new \OutOfBoundsException(sprintf('Invalid index "%s".', $index));
             }
 
+            if ($index > $oldSize) {
+                throw new \OutOfBoundsException(sprintf('Cannot insert index "%s" outside of collection.', $firstIndex));
+            }
+
             $slice = \is_array($slice) || $slice instanceof self ? $slice : [$slice];
             $sliceCount = \count($slice);