Please answer these questions before submitting your issue. Thanks!

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

package main

import (
    "encoding/asn1"
    "time"
)

type A struct {
    t time.Time
}

func main() {
    asn1.Marshal(A{})
}

What did you expect to see?

no panic.

What did you see instead?

panic: reflect.Value.Interface: cannot return value obtained from unexported field or method

goroutine 1 [running]:
panic(0xb14a0, 0xc420072080)
/Users/hiro/go/src/runtime/panic.go:527 +0x1a0
reflect.valueInterface(0xcdd20, 0xc42008a0e0, 0xb9, 0x133001, 0xcdd20, 0x1)
/Users/hiro/go/src/reflect/value.go:918 +0x1b5
reflect.Value.Interface(0xcdd20, 0xc42008a0e0, 0xb9, 0x130100, 0xcdd20)
/Users/hiro/go/src/reflect/value.go:907 +0x44
encoding/asn1.makeField(0xcdd20, 0xc42008a0e0, 0xb9, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/Users/hiro/go/src/encoding/asn1/marshal.go:582 +0x9c6
encoding/asn1.makeBody(0xbb180, 0xc42008a0e0, 0x99, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc42003bbe0, ...)
/Users/hiro/go/src/encoding/asn1/marshal.go:453 +0xa95
encoding/asn1.makeField(0xbb180, 0xc42008a0e0, 0x99, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc42008a0e0, ...)
/Users/hiro/go/src/encoding/asn1/marshal.go:596 +0x2b4
encoding/asn1.Marshal(0xbb180, 0xc42008a0e0, 0xbb180, 0xc42008a0e0, 0x0, 0x0, 0x0)
/Users/hiro/go/src/encoding/asn1/marshal.go:641 +0xe6
main.main()
/Users/hiro/a.go:13 +0x69
exit status 2

Does this issue reproduce with the latest release (go1.7.1)?

System details

go version devel +05cbf45 Sun Oct 16 13:47:13 2016 +0900 darwin/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/hiro/.go"
GORACE=""
GOROOT="/Users/hiro/go"
GOTOOLDIR="/Users/hiro/go/pkg/tool/darwin_amd64"
TERM="dumb"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/wq/dwn8hs0x7njbzty9f68y61700000gn/T/go-build126282944=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
uname -v: Darwin Kernel Version 15.6.0: Mon Aug 29 20:21:34 PDT 2016; root:xnu-3248.60.11~1/RELEASE_X86_64
ProductName:    Mac OS X
ProductVersion: 10.11.6
BuildVersion:   15G1004
lldb --version: lldb-360.1.50
gdb --version: GNU gdb (GDB) 7.11.1