diff --git a/doc/formats.txt b/doc/formats.txt
index 6864b05..f74f010 100644
--- a/doc/formats.txt
+++ b/doc/formats.txt
@@ -225,7 +225,7 @@ CATALOG
 	``CATALOG "<catalog_number>"``
 
 	``catalog_number``
-		13 digit number
+		an integer consisting of exactly 13 digits.
 
 CD_DA
 ~~~~~