Error trying to differentiate through a `Complex`

I need to treat a complex function f: C → C as a function from R^2 to R^2. I am doing this as follows:

julia 0.6> function realify(f)

               function g(x)
                   z = Complex(x[1], x[2])
                   z2 = f(z)
                   SVector(reim(z2))
               end

               return g

           end
realify (generic function with 1 method)

julia 0.6> using StaticArrays

julia 0.6> f(z) = z^3 - 1
f (generic function with 1 method)

julia 0.6> const g = realify(f)
g (generic function with 1 method)

julia 0.6> g(SVector(1, 2))
2-element SVector{2,Int64}:
 -12
  -2

When I try to differentiate this, I get the following error:

julia 0.6> using ForwardDiff

julia 0.6> ForwardDiff.jacobian(g, SVector(1, 2))
ERROR: MethodError: Cannot `convert` an object of type ForwardDiff.Dual{ForwardDiff.Tag{#g#1{#f},0x570e4f4889f9f966},Int64,2} to an object of type Signed
This may have arisen from a call to the constructor Signed(...),
since type constructors fall back to convert methods.
Stacktrace:
 [1] ^(::Complex{ForwardDiff.Dual{ForwardDiff.Tag{#g#1{#f},0x570e4f4889f9f966},Int64,2}}, ::Complex{ForwardDiff.Dual{ForwardDiff.Tag{#g#1{#f},0x570e4f4889f9f966},Int64,2}}) at ./complex.jl:625
 [2] f(::Complex{ForwardDiff.Dual{ForwardDiff.Tag{#g#1{#f},0x570e4f4889f9f966},Int64,2}}) at ./REPL[3]:1
 [3] (::#g#1{#f})(::SVector{2,ForwardDiff.Dual{ForwardDiff.Tag{#g#1{#f},0x570e4f4889f9f966},Int64,2}}) at ./REPL[1]:5
 [4] jacobian(::#g#1{#f}, ::SVector{2,Int64}) at /Users/dpsanders/.julia/v0.6/ForwardDiff/src/jacobian.jl:76

Reproduce the Julia 0.6 example with realify, f, g, and ForwardDiff.jacobian from the issue. Start at the jacobian entry point and follow the stack trace through the Complex exponentiation path in complex.jl. Done means the realified complex function can be differentiated without the reported conversion error.