Namespace linkage problem

We are using the RTS-kit to make a RTS game and have declared our own unit class that inherits from RTSUnit. We also have a class UnitData that inherits from RTSUnitData like this:

// Unit.h
class UnitData: public RTSUnitData
{
  private:
    typedef RTSUnitData Parent;
  public:
    //...
    DECLARE_CONOBJECT( UnitData );
};

// Unit.cc
IMPLEMENT_CO_DATABLOCK_V1( UnitData );

On startup we get the following error at the top of the console:

Error: cannot change namespace parent linkage for UnitData from SimObject to RTSUnitData.

I can't figure out why this error occures. Any ideas? Thanks!