Question: Flux through a cone

I'm trying to calculate flux through a cone but gets the following:

Flux(VectorField(`<,>`(x, y, z), cartesian[x, y, z]), Surface(`<,>`(x,y,z), x^2+y^2<=2*a,z = 2*a-sqrt(x^2+y^2)))
Error, (in VectorCalculus:-Flux) error with surface input

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