diff --git a/core/basepage.php b/core/basepage.php
index 2657ac80..5806e880 100644
--- a/core/basepage.php
+++ b/core/basepage.php
@@ -273,6 +273,7 @@ class BasePage
                 if (!is_null($this->filename)) {
                     header('Content-Disposition: ' . $this->disposition . '; filename=' . $this->filename);
                 }
+                assert($this->file, "file should not be null with PageMode::FILE");
 
                 // https://gist.github.com/codler/3906826
                 $size = filesize($this->file); // File size