diff --git a/files_opds/lib/meta.php b/files_opds/lib/meta.php
index 5b763be..817c691 100644
--- a/files_opds/lib/meta.php
+++ b/files_opds/lib/meta.php
@@ -91,7 +91,7 @@ class Meta
 				$meta['title'],
 				$meta['language'],
 				$meta['publisher'],
-				$meta['isbn'],
+				preg_replace('/[^0-9xX]/','',$meta['isbn']),
 				$meta['copyright'],
 				mb_strimwidth($meta['description'],0,2044,'...'),
 				$meta['subjects'],
@@ -110,7 +110,7 @@ class Meta
 				$meta['title'],
 				$meta['language'],
 				$meta['publisher'],
-				$meta['isbn'],
+				preg_replace('/[^0-9xX]/','',$meta['isbn']),
 				$meta['copyright'],
 				mb_strimwidth($meta['description'],0,2044,'...'),
 				$meta['subjects'],