I haven't looked into this, but removing the ,attr tag makes it work. Having the ,attr tag there doesn't make sense anyways, but I would expect it to return an error, not segfault.

What version of Go are you using (go version)?

$ go version
go version devel +7a89474fe1 Tue Jul 4 22:55:46 2017 -0500 linux/amd64

What operating system and processor architecture are you using (go env)?

$ go env
GOARCH="amd64"
GOBIN="/home/sam/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/sam"
GORACE=""
GOROOT="/home/sam/src/go"
GOTOOLDIR="/home/sam/src/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build927551460=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

package main

import (
	"encoding/xml"
	"log"
	"os"
)

type CancelError struct {
	XMLName xml.Name `error"`
	Type    struct {
		XMLName xml.Name `xml:"type,attr"`
	}
}

func main() {
	e := xml.NewEncoder(os.Stdout)
	if err := e.Encode(CancelError{}); err != nil {
		log.Fatal(err)
	}
}

Playground

What did you expect to see?

XML output to stdout or an error returned if the struct was not able to be marshaled. Maybe an "xml: unsupported type" error similar to trying to marshal:

type CancelError struct {
	XMLName xml.Name `xml:"urn:xmpp:err error"`
	Type    struct{} `xml:"type,attr"`
}

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0xffffffff addr=0x0 pc=0xf304a]

goroutine 1 [running]:
encoding/xml.lookupXMLName(0x1a2c60, 0x114ae0, 0x1280ea, 0x1)
	/usr/local/go/src/encoding/xml/typeinfo.go:246 +0x2ca
encoding/xml.structFieldInfo(0x1a2c60, 0x118400, 0x10529d94, 0x3, 0x0, 0x0, 0x1a2c60, 0x114ae0)
	/usr/local/go/src/encoding/xml/typeinfo.go:192 +0x5c0
encoding/xml.getTypeInfo(0x1a2c60, 0x118400, 0x115c60, 0x23, 0x1a2c00, 0x118400)
	/usr/local/go/src/encoding/xml/typeinfo.go:92 +0x3e0
encoding/xml.(*printer).marshalValue(0x1054e050, 0x118400, 0x105402e0, 0x99, 0x0, 0x0, 0x1054e050, 0x105402e0)
	/usr/local/go/src/encoding/xml/marshal.go:455 +0x2c0
encoding/xml.(*Encoder).Encode(0x1054e050, 0x118400, 0x105402e0, 0x105402e0, 0x10564000, 0x1000)
	/usr/local/go/src/encoding/xml/marshal.go:155 +0x80
main.main()
	/tmp/sandbox323847863/main.go:18 +0x120