cannot create and assign struct to interface

[nrwiersma]

The following program sample.go triggers a panic:

package main

type S struct {
	a int
}

func main() {
	var i interface{} = S{a: 1}

	s, ok := i.(S)
	if !ok {
		println("bad")
		return
	}
	println(s.a)
}

Expected result:

$ go run ./sample.go
1

Got:

$ yaegi ./sample.go
_test/test.go:8:8: panic
reflect.Set: value of type struct { Xa int } is not assignable to type interp.valueInterface
goroutine 1 [running]:
runtime/debug.Stack(0x1, 0xc0002f0600, 0x40)
	/usr/local/Cellar/go/1.14.4/libexec/src/runtime/debug/stack.go:24 +0x9d
github.com/containous/yaegi/interp.(*Interpreter).Eval.func1(0xc0001adde0)
	/Users/nrwiersma/Go/src/github.com/containous/yaegi/interp/interp.go:333 +0xbc
panic(0x191b6c0, 0xc00009a830)
	/usr/local/Cellar/go/1.14.4/libexec/src/runtime/panic.go:969 +0x166
github.com/containous/yaegi/interp.runCfg.func1(0xc00036c0a0, 0xc000375400)
	/Users/nrwiersma/Go/src/github.com/containous/yaegi/interp/run.go:116 +0x20c
panic(0x191b6c0, 0xc00009a830)
	/usr/local/Cellar/go/1.14.4/libexec/src/runtime/panic.go:969 +0x166
reflect.Value.assignTo(0xc000096360, 0xc000369c30, 0x199, 0x1a57815, 0xb, 0x19a2b80, 0x0, 0xc000324840, 0xc000369c58, 0xc00037e000)
	/usr/local/Cellar/go/1.14.4/libexec/src/reflect/value.go:2403 +0x426
reflect.Value.Set(0x19a2b80, 0xc00009ee40, 0x199, 0xc000096360, 0xc000369c30, 0x199)
	/usr/local/Cellar/go/1.14.4/libexec/src/reflect/value.go:1532 +0xbd
github.com/containous/yaegi/interp.doCompositeSparse.func1(0xc00036c0a0, 0xc0000a4980)
	/Users/nrwiersma/Go/src/github.com/containous/yaegi/interp/run.go:2116 +0x264
github.com/containous/yaegi/interp.runCfg(0xc000375400, 0xc00036c0a0)
	/Users/nrwiersma/Go/src/github.com/containous/yaegi/interp/run.go:122 +0x6d
github.com/containous/yaegi/interp.(*Interpreter).run(0xc0000be1a0, 0xc000374a00, 0xc00036c000)
	/Users/nrwiersma/Go/src/github.com/containous/yaegi/interp/run.go:100 +0x239
github.com/containous/yaegi/interp.(*Interpreter).Eval(0xc0000be1a0, 0xc0002cce60, 0x9b, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/nrwiersma/Go/src/github.com/containous/yaegi/interp/interp.go:409 +0x52b
main.main()
	/Users/nrwiersma/Go/src/github.com/containous/yaegi/cmd/yaegi/yaegi.go:159 +0x65d