Question: Error message in dsolve/numeric

I want to solve the transient heat diffusion equation in a semi-infinite solid, both analyticall and numerically:

BVP1 := { diff( theta(eta), eta, eta) +2 * eta * diff(theta(eta), eta), theta(infinity) = 0, theta(0) = 1 };
sol1 := dsolve(BVP1);
                   theta(eta) = 1 - erf(eta)
which is the correct exact self-similar solution.
dsolve(BVP1, numeric) ;
and I got this error message:
Error, (in dsolve/numeric/bvp/convertsys) too few boundary conditions: expected 3, got 2
Why does Maple need three conditions and what is this extra condition? 
Thanks for your help.
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