  It has been proposed to determine the CKM matrix element |V_ub| in a
model-independent way from a combination of rare and semileptonic B and D
decays near the zero recoil point. An essential ingredient in such a
determination is a heavy quark symmetry relation connecting the form-factors
appearing in B -> K* e+e- to semileptonic form factors relevant for B -> rho
e\nu. We estimate the leading corrections to this symmetry relation, of order
\alpha_s(m_b) and \Lambda/m_b, pointing out that they can be as large as 20%,
depending on the value of the matrix element of a dimension-4 operator.
Dimensional analysis estimates of this matrix element give a corresponding
uncertainty in |V_ub| of the order of a few percent.
