diff --git a/resources/dtd/database.dtd b/resources/dtd/database.dtd
index 767a230ac6..684ec39d5e 100644
--- a/resources/dtd/database.dtd
+++ b/resources/dtd/database.dtd
@@ -135,7 +135,8 @@ PHP class or method name.
 <!ELEMENT reference EMPTY>
 <!ATTLIST reference
   local CDATA #REQUIRED
-  foreign CDATA #REQUIRED
+  foreign CDATA #IMPLIED
+  value CDATA #IMPLIED
 >
 
 <!ELEMENT index (index-column+,vendor*)>
diff --git a/resources/xsd/database.xsd b/resources/xsd/database.xsd
index 66dff53243..237d70204a 100644
--- a/resources/xsd/database.xsd
+++ b/resources/xsd/database.xsd
@@ -294,7 +294,8 @@
 
     <xs:complexType name="reference">
         <xs:attribute name="local" type="column_name" use="required"/>
-        <xs:attribute name="foreign" type="column_name" use="required"/>
+        <xs:attribute name="foreign" type="column_name" use="optional"/>
+        <xs:attribute name="value" type="xs:string" use="required"/>
     </xs:complexType>
 
     <xs:complexType name="behavior">