Generating Random String of Numbers and Letters Using Go's "testing/quick" Package

I've been breaking my head over this for a few days now and can't seem to be able to figure it out. Perhaps it's glaringly obvious, but I don't seem to be able to spot it. I've read up on all the basics of unicode, UTF-8, UTF-16, normalisation, etc, but to no avail. Hopefully somebody's able to help me out here...

I'm using Go's Value function from the testing/quick package to generate random values for the fields in my data structs, in order to implement the Generator interface for the structs in question. Specifically, given a Metadata struct, I've defined the implementation as follows:

func (m *Metadata) Generate(r *rand.Rand, size int) (value reflect.Value) {
    value = reflect.ValueOf(m).Elem()
    for i := 0; i < value.NumField(); i++ {
        if t, ok := quick.Value(value.Field(i).Type(), r); ok {
            value.Field(i).Set(t)
        }
    }
    return
}

Now, in doing so, I'll end up with both the receiver and the return value being set with random generated values of the appropriate type (strings, ints, etc. in the receiver and reflect.Value in the returned reflect.Value).

Now, the implementation for the Value function states that it will return something of type []rune converted to type string. As far as I know, this should allow me to then use the functions in the runes, unicode and norm packages to define a filter which filters out everything which is not part of 'Latin', 'Letter' or 'Number'. I defined the following filter which uses a transform to filter out letters which are not in those character rangetables (as defined in the unicode package):

func runefilter(in reflect.Value) (out reflect.Value) {
    out = in // Make sure you return something
    if in.Kind() == reflect.String {
        instr := in.String()
        t := transform.Chain(norm.NFD, runes.Remove(runes.NotIn(rangetable.Merge(unicode.Letter, unicode.Latin, unicode.Number))), norm.NFC)
        outstr, _, _ := transform.String(t, instr)
        out = reflect.ValueOf(outstr)
    }
    return
}

Now, I think I've tried just about anything, but I keep ending up with a series of strings which are far from the Latin range, e.g.:

𥗉똿穊

𢷽嚶
秓䝏小𪖹䮋
𪿝ท솲
𡉪䂾

ʋ𥅮ᦸ
堮𡹯憨𥗼𧵕ꥆ
𢝌𐑮𧍛併怃𥊇
鯮
𣏲𝐒
⓿ꐠ槹𬠂黟
𢼭踁퓺𪇖
俇𣄃𔘧

𢝶
𝖸쩈𤫐𢬿詢𬄙

𫱘𨆟𑊙
欓

So, can anybody explain what I'm overlooking here and how I could instead define a transformer which removes/replaces non-letter/number/latin characters so that I can use the Value function as intended (but with a smaller subset of 'random' characters)?

Thanks!