Problem with subst() after diff()

This little script

assume(l>0);
assume(t>0);
Q:G*M*l*v*t/(2*(l^2+(v*t)^2)^(3/2));
depends(v,t);
D2Q:diff(Q,t,2);
subst(0,t,D2Q);

crashes on the last line ...

..
..
(%i4) depends(v,t)
(%o4) [v(t)]
(%i5) D2Q:diff(Q,t,2)
(%o5) [ snipped long expression]
(%i6) subst(0,t,D2Q)
Attempt to differentiate with respect to a number:
0
 -- an error.  To debug this try debugmode(true);

I'm baffled. I thought the differentiating was done. Any help appreciated.

Lut Mentz