This library lacks user input validation

Hi.

If I do:
```javascript
formData.append("key", undefinedValue)
```
it causes a crash at :

![Image](https://github.com/user-attachments/assets/76153315-41b2-48f6-a7ef-add890235563)

![Image](https://github.com/user-attachments/assets/645c0314-b8f4-4476-a023-10478b9f9554)

`value` is not defined, so a crash occurs. This is basic user input validation :) 