diff --git a/svrl.rnc b/svrl.rnc
index 39f7800..f6e0770 100644
--- a/svrl.rnc
+++ b/svrl.rnc
@@ -33,11 +33,15 @@ schematron-output =
         attribute phase { xsd:NMTOKEN }?,
         attribute schemaVersion { text }?,
         attribute schematronEdition { text }?,
-        error*,
-        human-text*,
-        ns-prefix-in-attribute-values*,
-        (active-pattern-rule-set,
-         (fired-rule, (failed-assert | successful-report)*)*)+
+        (
+            (error+, human-text*)
+            |
+            (error*,
+            human-text*,
+            ns-prefix-in-attribute-values*,
+            (active-pattern-rule-set,
+             (fired-rule, (failed-assert | successful-report)*)*)+)
+        )
     }
 # only namespaces from sch:ns need to be reported 
 ns-prefix-in-attribute-values =