diff --git a/grammars/basicdoc.rnc b/grammars/basicdoc.rnc
index cd8c66f..72c6676 100644
--- a/grammars/basicdoc.rnc
+++ b/grammars/basicdoc.rnc
@@ -881,7 +881,7 @@ HyperlinkAttributes =
     ## The location or online identifier of the external document or resource
     attribute target { xsd:anyURI },
     ## The type of Reference Element, prescribing how it is to be rendered
-    attribute type { ReferenceFormat },
+    attribute type { ReferenceFormat }?,
     ## Alternate text, used for accessibility
     attribute alt { text }?
 
diff --git a/grammars/basicdoc.rng b/grammars/basicdoc.rng
index f07f4db..08b0241 100644
--- a/grammars/basicdoc.rng
+++ b/grammars/basicdoc.rng
@@ -1635,10 +1635,12 @@ which can be bookmarks as well as block or section references</a:documentation>
       <a:documentation>The location or online identifier of the external document or resource</a:documentation>
       <data type="anyURI"/>
     </attribute>
-    <attribute name="type">
-      <a:documentation>The type of Reference Element, prescribing how it is to be rendered</a:documentation>
-      <ref name="ReferenceFormat"/>
-    </attribute>
+    <optional>
+      <attribute name="type">
+        <a:documentation>The type of Reference Element, prescribing how it is to be rendered</a:documentation>
+        <ref name="ReferenceFormat"/>
+      </attribute>
+    </optional>
     <optional>
       <attribute name="alt">
         <a:documentation>Alternate text, used for accessibility</a:documentation>
diff --git a/grammars/relaton-models b/grammars/relaton-models
index 2e870b2..2c42b17 160000
--- a/grammars/relaton-models
+++ b/grammars/relaton-models
@@ -1 +1 @@
-Subproject commit 2e870b264dc853335cb1b42e3ab5713b47101cee
+Subproject commit 2c42b172c341570545f7925702687c91e41c659e