diff --git a/include/xo/alloc/Object.hpp b/include/xo/alloc/Object.hpp
index c1217418..742fba4c 100644
--- a/include/xo/alloc/Object.hpp
+++ b/include/xo/alloc/Object.hpp
@@ -118,7 +118,7 @@ namespace xo {
 
         /** tagged pointer with runtime type information
          **/
-        virtual TaggedPtr self_tp() const = 0;
+        virtual TaggedPtr self_tp() const;
 
         /** print on stream @p os **/
         virtual void display(std::ostream & os) const = 0;
diff --git a/src/alloc/Object.cpp b/src/alloc/Object.cpp
index 75abec14..ab23b76a 100644
--- a/src/alloc/Object.cpp
+++ b/src/alloc/Object.cpp
@@ -20,9 +20,18 @@ operator new (std::size_t z, const xo::Cpof & cpof)
 }
 
 namespace xo {
+    using xo::reflect::TaggedPtr;
+
     gc::IAlloc *
     Object::mm = nullptr;
 
+    TaggedPtr
+    Object::self_tp() const
+    {
+        assert(false);
+        return TaggedPtr::universal_null();
+    }
+
     IObject *
     Object::_forward(IObject * src,
                      gc::IAlloc * gc)