diff --git a/DOMTEST b/DOMTEST
index 20a3e5e..2a22049 100644
--- a/DOMTEST
+++ b/DOMTEST
@@ -812,7 +812,6 @@ No entity resolver registered.
 775/808 prefix07.xml
 776/808 prefix08.xml
 777/808 prefix09.xml
-TEST FAILED: expected exception NAMESPACE_ERR
 778/808 prefix10.xml
 779/808 prefix11.xml
 780/808 publicId01.xml
@@ -846,4 +845,4 @@ implementationAttribute expandEntityReferences not supported, skipping test
 803/808 setNamedItemNS04.xml
 804/808 setNamedItemNS05.xml
 805/808 systemId01.xml
-4/763 tests failed; 45 tests were skipped
\ No newline at end of file
+3/763 tests failed; 45 tests were skipped
\ No newline at end of file
diff --git a/dom/dom-impl.lisp b/dom/dom-impl.lisp
index ff94042..489a7d3 100644
--- a/dom/dom-impl.lisp
+++ b/dom/dom-impl.lisp
@@ -50,6 +50,10 @@
 		      (dom:namespace-uri node)))
   (setf (slot-value node 'prefix) newval))
 
+(defmethod (setf dom:prefix) :before (newval (node attribute))
+  (when (rod= (dom:node-name node) #"xmlns")
+    (dom-error :NAMESPACE_ERR "must not change xmlns attribute prefix")))
+
 (defmethod (setf dom:prefix) :after (newval (node attribute))
   (setf (slot-value node 'name)
 	(concatenate 'rod newval #":" (dom:local-name node))))