(!) Plugin svelte: No custom element 'tag' option was specified

I try to compile a Svelte component into a web component.

1. added the option <svelte:options tag="date-picker" immutable={true}/>
2. added customElement: true in rollup to plugins: [ svelte()
3. run npm run build

But I keep having the message:

(!) Plugin svelte: No custom element 'tag' option was specified. To automatically register a custom element, specify a name with a hyphen in it, e.g. . To hide this warning, use

What am I doing wrong?

My index.js file:

export { default as default } from './DatePicker.svelte';

DatePicker.svelte file:

<svelte:options tag="date-picker" immutable={true}/>

<script>
  /* code */

The rollup file:

import svelte from 'rollup-plugin-svelte';
import resolve from '@rollup/plugin-node-resolve';
import pkg from './package.json';

const name = pkg.name
    .replace(/^(@\S+\/)?(svelte-)?(\S+)/, '$3')
    .replace(/^\w/, m => m.toUpperCase())
    .replace(/-\w/g, m => m[1].toUpperCase());

export default {
    input: 'src/index.js',
    output: [
        { file: pkg.module, 'format': 'es' },
        { file: pkg.main, 'format': 'umd', name }
    ],
    plugins: [
        svelte({
            customElement: true
        }),
        resolve()
    ]
};

It seems that if your project consists of a single component, the message doesn't appear. See issue here:

https://github.com/sil-vio/svelte-web-components/issues/2