Why can't COM client applications access objects added to containers in my COM .DLL using AddObject() method?

Eg. compile the following prg add into a project and and build COM DLL

Define Class TopLevel As Custom OLEPublic

  Function TestMethod
    This.AddObject("Test","ChildClass")
  EndFunc

EndDefine

Define Class ChildClass As Custom OLEPublic

EndDefine

Attempting to access this child object gives an error.
Eg.

loCOMServer = CreateObject("ComServer.TopLevel")
loCOMServer.TestMethod()
loCOMServer.Test    && Gives error

From searching google groups this seems to be because there is not an interface defined for the child object. However I thought as the COM .DLL created by VFP uses IDispatch (is this the same as late-binding?) rather than the TLB this wouldn't matter.