ICS with diff

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I'm trying to solve a very easy ODE, with ics including diff:

sage: m, t, g = var('m t g')
sage: z = function('z')(t)
sage: DE = diff(z, t,2)+m*g
sage: sol = desolve(DE,[z,t])
sage: z0, v0 = var('z0 v0')
sage: sol_ics = desolve(DE, [z, t], ics=[0, z0, diff(sol, t)(t=0), v0])
sage: sol_ics
-1/2*g*m*t^2 + 1/2*(_K2^2*g*m + 2*v0 - 2*z0)*t/_K2 + z0

It does not seem to work well... What am I missing?