diff --git a/src/db/Command.php b/src/db/Command.php
index 99dda764882..0291c524295 100644
--- a/src/db/Command.php
+++ b/src/db/Command.php
@@ -117,7 +117,9 @@ public function upsert($table, $insertColumns, $updateColumns = true, $params =
 
         // todo: hack for BC with our old upsert() method. Remove in Craft 4
         // Merge any updateColumn data into insertColumns
-        $insertColumns = array_merge($updateColumns, $insertColumns);
+        if (is_array($updateColumns)) {
+            $insertColumns = array_merge($updateColumns, $insertColumns);
+        }
 
         parent::upsert($table, $insertColumns, $updateColumns, $params);
         return $this;