diff --git a/htdocs/bookcal/class/availabilities.class.php b/htdocs/bookcal/class/availabilities.class.php
index ad34e08abcd..6a4553e15f2 100644
--- a/htdocs/bookcal/class/availabilities.class.php
+++ b/htdocs/bookcal/class/availabilities.class.php
@@ -975,7 +975,7 @@ class Availabilities extends CommonObject
 				$dir = dol_buildpath($reldir."core/modules/bookcal/");
 
 				// Load file with numbering class (if found)
-				$mybool |= @include_once $dir.$file;
+				$mybool = ((bool) @include_once $dir.$file) || $mybool;
 			}
 
 			if ($mybool === false) {