[sigc] derive from sigc::trackable directly?

[Dick Eimers <d eimers marin nl>]

sigc::trackable is the base class for objects with auto-disconnection.

I have a hierarchy like 'class Base', 'class DerivedX : public Base' and
'class DerivedY : public Base'. 

The auto-disconnection seems to work fine when both DerivedX and
DerivedY derive from sigc::trackable directly (as in 'class DerivedY :
public Base, public sigc::trackable' etc.), however I've encountered
some issues (segfaults) when Base derives from sigc::trackable and
DerivedX and DerivedY don't (not directly that is). 

Is it necessary that every class that desires auto-disconnection to
derive from sigc::trackable directly? Why?