"invalid memory address" when calling Go from Python

I'm trying to extend the code in Calling Go Functions from Other Languages. I'd like see how a function returning an error can be handled. See the code below, I get panic: runtime error: invalid memory address or nil pointer dereference when running the Python code. Any ideas on how to fix this?

gmath.go

<!-- language: lang-golang -->

package main

import (
	"fmt"
)

import "C"

//export Div
func Div(x, y float64) (float64, error) {
	fmt.Printf("Div called: x=%f, y=%f\n", x, y)
	if y == 0 {
		return 0, fmt.Errorf("ZeroDivisionError")
	}
	return x / y, nil
}

func main() {}

gmath.py

<!-- language: lang-py -->

import ctypes


class GoInterface(ctypes.Structure):
    _fields_ = [
        ('t', ctypes.c_void_p),
        ('v', ctypes.c_void_p),
    ]


class DivRet(ctypes.Structure):
    _fields_ = [
        ('result', ctypes.c_float),
        ('error', GoInterface),
    ]


lib = ctypes.cdll.LoadLibrary('./gmath.so')
lib.Div.argtypes = [ctypes.c_double, ctypes.c_double]
lib.Div.restype = DivRet

r = lib.Div(1.2, 2.3)

Example run

<!-- language: lang-none -->

$ make
go build -o gmath.so -buildmode=c-shared gmath.go
$ python gmath.py
Div called: x=1.200000, y=2.300000
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x17 pc=0x7fbe5fda928d]

goroutine 17 [running, locked to thread]:
main._cgoexpwrap_448ea9090bef_Div.func1(0xc42003cea8)
    command-line-arguments/_obj/_cgo_gotypes.go:46 +0x42
main._cgoexpwrap_448ea9090bef_Div(0x3ff3333333333333, 0x4002666666666666, 0x3fe0b21642c8590b, 0x0, 0x0)
    command-line-arguments/_obj/_cgo_gotypes.go:48 +0xaa
[1]    29009 abort (core dumped)  python gmath.py

It appears that the error part of result is the initial issue. If I change error to a string and return the (float64, *C.char), it works.