diff --git a/python/core/qgsvectordataprovider.sip b/python/core/qgsvectordataprovider.sip
index 5e06c9947d5..ce47a933bd9 100644
--- a/python/core/qgsvectordataprovider.sip
+++ b/python/core/qgsvectordataprovider.sip
@@ -141,7 +141,7 @@ class QgsVectorDataProvider : QgsDataProvider
        *
        * Default implementation simply iterates the features 
        */
-      virtual void uniqueValues(int index, QList<QVariant> &uniqueValues);
+      virtual void uniqueValues(int index, QList<QVariant> &uniqueValues /Out/);
 
       /**
        * Adds a list of features