Panic when comparing two maps

This program

package main

import (
    "database/sql"

    "github.com/go-test/deep"
)

type NullString struct{ sql.NullString }

func main() {
    a := map[string]interface{}{
        "a": NullString{sql.NullString{"foo", true}},
    }
    b := map[string]interface{}{
        "a": sql.NullString{"foo", true},
    }
    deep.Equal(b, a)
}

panics with

panic: reflect: Field index out of range

goroutine 1 [running]:
reflect.Value.Field(0x10e1880, 0xc42000a060, 0x99, 0x1, 0x10d1a60, 0xc42000a090, 0x81)
    /usr/local/Cellar/go/1.9.2/libexec/src/reflect/value.go:769 +0x14c
github.com/go-test/deep.(*cmp).equals(0xc42003de98, 0x10db460, 0xc42000e2c0, 0x94, 0x10db460, 0xc42000e2d0, 0x94, 0x1)
    /Users/mibk/src/github.com/go-test/deep/deep.go:179 +0x648
github.com/go-test/deep.(*cmp).equals(0xc42003de98, 0x10dd240, 0xc4200721e0, 0x15, 0x10dd240, 0xc4200721b0, 0x15, 0x0)
    /Users/mibk/src/github.com/go-test/deep/deep.go:225 +0x1810
github.com/go-test/deep.Equal(0x10dd240, 0xc4200721e0, 0x10dd240, 0xc4200721b0, 0xc4200801a8, 0xc42003df70, 0x1004324)
    /Users/mibk/src/github.com/go-test/deep/deep.go:77 +0x284
main.main()
    /Users/mibk/src/github.com/mibk/junk/deep/main.go:18 +0x211
exit status 2

Note that if I flip the arguments (deep.Equal(a, b)), the program works just fine.

go version go1.9.2 darwin/amd64
GOARCH="amd64"
GOOS="darwin"

go-test / deep

Panic when comparing two maps #9