SeleniumHQ/selenium · error · ValueError

argument can not be null

Error message

argument can not be null

What it means

add_argument rejects falsy values (None, empty string, 0) because the code uses a truthiness check `if argument:`. Any empty/None argument raises ValueError. Note that a string of only whitespace (' ') is truthy and would be accepted, so this guards emptiness, not validity.

Source

Thrown at py/selenium/webdriver/common/options.py:409

    def __init__(self) -> None:
        super().__init__()
        self._arguments: list[str] = []

    @property
    def arguments(self):
        """Returns a list of arguments needed for the browser."""
        return self._arguments

    def add_argument(self, argument: str) -> None:
        """Adds an argument to the list.

        Args:
            argument: Sets the arguments
        """
        if argument:
            self._arguments.append(argument)
        else:
            raise ValueError("argument can not be null")

    def ignore_local_proxy_environment_variables(self) -> None:
        """Ignore HTTP_PROXY and HTTPS_PROXY environment variables.

        This method is deprecated; use a Proxy instance with ProxyType.DIRECT instead.
        """
        warnings.warn(
            "using ignore_local_proxy_environment_variables in Options has been deprecated, "
            "instead, create a Proxy instance with ProxyType.DIRECT to ignore proxy settings, "
            "pass the proxy instance into a ClientConfig constructor, "
            "pass the client config instance into the Webdriver constructor",
            DeprecationWarning,
            stacklevel=2,
        )

        super().ignore_local_proxy_environment_variables()

    def to_capabilities(self):

View on GitHub (pinned to aa36b38e69)

Solutions

1. Filter falsy entries before adding: [a for a in args if a].
2. Ensure the argument is a non-empty string.

Example fix

# before
for a in raw_args.split(' '):
    options.add_argument(a)  # '' from trailing space -> ValueError

# after
for a in raw_args.split(' '):
    if a:
        options.add_argument(a)

for a in args:
    if not a:
        continue
    options.add_argument(a)
# or filter upfront
clean_args = [a for a in args if a]

def is_valid_argument(v) -> bool:
    return isinstance(v, str) and len(v.strip()) > 0

try:
    options.add_argument(a)
except ValueError:
    pass  # skip falsy/empty arguments

When it happens

Trigger: Calling options.add_argument(None), options.add_argument(''), or options.add_argument(0). Common when arguments come from a list that contains an empty entry, e.g. splitting an arg string that yields ''.

Common situations: Parsing args from a config/env var that is unset. Splitting '--flag ' which yields an empty trailing element. Building args programmatically and accidentally appending None.

Related errors

• argument can not be null
• Strategy can only be one of the following: normal, eager, no
• Behavior can only be one of the following: dismiss, accept,
• Timeout keys can only be one of the following: implicit, pag
• android_package must be passed in

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/5140b3646fe984fe. Report an issue: GitHub.