Error when trying to encode Vietnamese chars

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Error when trying to encode Vietnamese chars

> InvalidCharacterError
> at /home/runner/invalidlatin/node_modules/base-64/base64.js:23:36
> at Object. (/home/runner/invalidlatin/node_modules/base-64/base64.js:165:2)
> at Module._compile (internal/modules/cjs/loader.js:1133:30)
> at Object.Module._extensions..js (internal/modules/cjs/loader.js:1153:10)
> at Module.load (internal/modules/cjs/loader.js:977:32)
> at Function.Module._load (internal/modules/cjs/loader.js:877:14)
> at Module.require (internal/modules/cjs/loader.js:1019:19)
> at /home/runner/invalidlatin/index.js:1:17
> at Script.runInContext (vm.js:131:20)
>

Code
```
const base64 = require("base-64") ;

const str= "Những hình dạng của kinh tế hậu COVID-19 - Tuổi Trẻ Online";

console.log(base64.encode(str))

```

Error Example:
https://repl.it/@SoubhikChatterj/invalidlatin

Reproduce the failure from the index.js example and inspect the stack-trace location in node_modules/base-64/base64.js. Determine the expected handling of Vietnamese characters, then add a regression test covering the reported string and verify that encoding no longer raises InvalidCharacterError.

Written by the indexing model from the issue text.