Mean of ItoProcess

I’ve defined the following ItoProcess

proc = ItoProcess[   {    \d bj[t] == bk[t] \d pj[t],    \d bk[t] == bj[t] \d pj[t]    },   bk[t],   {{bj, bk}, {bj0, bk0}},   t,   pj ~ GeometricBrownianMotionProcess[u, s, pj0]   ] 

proc evaluates to:

ItoProcess[{   {u X[t], u X[t] bk[t], u X[t] bj[t]},   {{s X[t]}, {s X[t] bk[t]}, {s X[t] bj[t]}},   bk[t]},    {{X, bj, bk}, {pj0, bj0, bk0}},    {t,0}] 

However Mean[proc[t]] or anything else I ask about proc returns unevaluated. I’m obviously doing something wrong. Is there a way to ask Mathematica for more insight about what the error could be?