diff --git a/dev/Debug.php b/dev/Debug.php
index c17d68c82..46a0c430f 100644
--- a/dev/Debug.php
+++ b/dev/Debug.php
@@ -347,6 +347,7 @@ class Debug {
 				);
 				if(file_exists($errorFilePath)) {
 					$content = file_get_contents(ASSETS_PATH . "/error-$statusCode.html");
+					if(!headers_sent()) header('Content-Type: text/html');
 					// $BaseURL is left dynamic in error-###.html, so that multi-domain sites don't get broken
 					echo str_replace('$BaseURL', Director::absoluteBaseURL(), $content);
 				}