diff --git a/src/main/resources/com/solab/iso8583/parse/j8583.dtd b/src/main/resources/com/solab/iso8583/parse/j8583.dtd
index 833fc14..aadb46e 100644
--- a/src/main/resources/com/solab/iso8583/parse/j8583.dtd
+++ b/src/main/resources/com/solab/iso8583/parse/j8583.dtd
@@ -2,6 +2,7 @@
 
 <!ELEMENT header ( #PCDATA ) >
 <!ATTLIST header type NMTOKEN #REQUIRED >
+<!ATTLIST header ref NMTOKEN #IMPLIED >
 
 <!ELEMENT template ( field* ) >
 <!ATTLIST template type NMTOKEN #REQUIRED >