diff --git a/src/mobility/position-allocator.cc b/src/mobility/position-allocator.cc
index cdd093d50..72610e93b 100644
--- a/src/mobility/position-allocator.cc
+++ b/src/mobility/position-allocator.cc
@@ -73,6 +73,8 @@ ListPositionAllocator::GetNext (void) const
   return v;
 }
 
+NS_OBJECT_ENSURE_REGISTERED (GridPositionAllocator);
+
 TypeId 
 GridPositionAllocator::GetTypeId (void)
 {