Take2 on last, a=chris

diff --git a/src/library/PartitionDocumentBundle.php b/src/library/PartitionDocumentBundle.php
index 9af277352..7e3ed417b 100644
--- a/src/library/PartitionDocumentBundle.php
+++ b/src/library/PartitionDocumentBundle.php
@@ -345,6 +345,7 @@ class PartitionDocumentBundle
             $index = $this->index_cache[$partition][0];
             $this->index_cache[$partition][1] = $this->index_cache['time'];
         } else {
+            $this->index_cache ??= [];
             if (count($this->index_cache) > $this->index_cache_size) {
                 $oldest_partition = -1;
                 $oldest_time = 2 * time();