Is it possible to use this package with piecewise functions (e.g., Huber loss)?

@variables a
function huber(a)
    δ = 1.0
    if abs(a) <= δ
        return 0.5*a^2
    else
        return δ*(abs(a) - 0.5*δ)
    end
end
da = huber(a)

This errors on the last line:

TypeError: non-boolean (Operation) used in boolean context