SeleniumHQ/selenium · error · ValueError

argument can not be null

Error message

argument can not be null

What it means

`add_extension` rejects a falsy argument (empty string, None, etc.) with `ValueError("argument can not be null")`. An empty/None extension path is meaningless to chromedriver, so the library fails fast rather than silently queuing nothing. Note the parameter is typed `str` but the guard accepts any falsy value.

Source

Thrown at py/selenium/webdriver/chromium/options.py:101

            with open(extension, "rb") as f:
                encoded_extensions.append(_decode(f))

        return encoded_extensions + self._extensions

    def add_extension(self, extension: str) -> None:
        """Add the path to an extension to be extracted to ChromeDriver.

        Args:
            extension: Path to the *.crx file.
        """
        if extension:
            extension_to_add = os.path.abspath(os.path.expanduser(extension))
            if os.path.exists(extension_to_add):
                self._extension_files.append(extension_to_add)
            else:
                raise OSError("Path to the extension doesn't exist")
        else:
            raise ValueError("argument can not be null")

    def add_encoded_extension(self, extension: str) -> None:
        """Add Base64-encoded string with extension data to be extracted to ChromeDriver.

        Args:
            extension: Base64 encoded string with extension data.
        """
        if extension:
            self._extensions.append(extension)
        else:
            raise ValueError("argument can not be null")

    @property
    def experimental_options(self) -> dict:
        """Returns a dictionary of experimental options for chromium."""
        return self._experimental_options

    def add_experimental_option(self, name: str, value: str | int | dict | list[str]) -> None:

View on GitHub (pinned to aa36b38e69)

Solutions

1. Guard before calling: `if ext_path: options.add_extension(ext_path)`.
2. Validate config early and supply a real path or skip.
3. Use `add_encoded_extension` if you have raw base64 data instead.

Example fix

// before
options.add_extension(config.get("crx_path"))  # None when unset -> ValueError
// after
if crx := config.get("crx_path"):
    options.add_extension(crx)

ext_path = config.get("crx_path") or ""
if ext_path:
    options.add_extension(ext_path)

def is_non_empty(value) -> bool:
    return bool(value)

try:
    options.add_extension(ext_path)
except ValueError:
    # ext_path was empty; skip or assign a real path
    ...

When it happens

Trigger: `options.add_extension("")`, `options.add_extension(None)`, or `options.add_extension(0)`. Often the result of a variable that was never assigned a path.

Common situations: Config-driven setups where the extension path is optional and left empty. Reading from an env var that is unset (defaults to empty string). Looping over a list that contains an empty entry.

Related errors

• Path to the extension doesn't exist
• could not find extension at #{path.inspect}
• Couldn't find manifest.json in ${extension}
• Could not find add-on ID for ${extension}
• Binary Location Must be a String

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/a6227bcedd659d57. Report an issue: GitHub.