--- a/src/mobility/hierarchical-mobility-model.cc	Sun Feb 17 00:46:24 2008 +0100
+++ b/src/mobility/hierarchical-mobility-model.cc	Sun Feb 17 04:38:34 2008 +0100
@@ -31,8 +31,10 @@
     .SetParent<MobilityModel> ()
     .AddConstructor<HierarchicalMobilityModel> ()
     .AddParameter ("child", "The child mobility model.",
+                   Ptr<MobilityModel> (0),
                    MakePtrParamSpec (&HierarchicalMobilityModel::SetChild))
     .AddParameter ("parent", "The parent mobility model.",
+                   Ptr<MobilityModel> (0),
                    MakePtrParamSpec (&HierarchicalMobilityModel::SetParent))
     ;
   return tid;