Typed map does not support indexing

I want to extend go-validator to return a better type:

type Error map[string][]error

// Will output the first error when stringified (e.g. for json response).
func (err Error) Error() string {
	for k, errs := range err {
		return fmt.Sprintf("%s value had %s", k, errs[0].Error())
	}

	return "no error"
}

func Validate(v interface{}) error {
	if ok, errs := DefaultValidator.Validate(v); !ok {
		return Error(errs)
	}

	return nil
}

As I am now writing tests for this I ran into the problem that it the typed map Error seems to have lost it indexing capabilities:

err = Validate(Value{
	Foo:      "bar",
	Email:    "foo@bar.me",
	Required: "1234",
})
c.Check(err, check.IsNil)

err, ok := Validate(Value{}).(Error)
c.Check(ok, check.Equals, true)
c.Check(err["Foo"], check.DeepEquals, []error{validator.ErrMin})
c.Check(err["Email"], check.DeepEquals, []error{validator.ErrInvalid})
c.Check(err["Required"], check.DeepEquals, []error{validator.ErrZeroValue})

Returns:

model/validation/validation_test.go:42: invalid operation: err["Foo"] (type error does not support indexing)
model/validation/validation_test.go:43: invalid operation: err["Email"] (type error does not support indexing)
model/validation/validation_test.go:44: invalid operation: err["Required"] (type error does not support indexing)

I also tried to cast to type map[string][]error but got "impossible type assertion".

What is wrong with my approach? How can I get indexing back to work?

It seems like your err variable is initialised with error type. When you do

err, ok := Validate(Value{}).(Error)

you merely check if err is actually an Error. If you change err there to say errs, it should work.

Playground example: http://play.golang.org/p/ljNroPzVbd