LastModified标头未出现

ctx.Response.Header.SetLastModified(changed)
if ctx.IfModifiedSince(changed) {
    buf = data
} else {
    ctx.SetStatusCode(fasthttp.StatusNotModified)
}

ctx is *fasthttp.RequestCtx.

I need a Last-Modified header in my response but I have only:

HTTP/1.1 200 OK
Content-Type: application/json

What am i doing wrong?