def check_arg_type(arg_name: str, arg_value: str):
    """
        Checks that RST Threat Feed API parameters are valid.

        Args:
          arg_name (str): paramater name
          arg_value (str): paramater value to verify
        Returns:
          (str): a null string means OK while any text is an error
        """
    output = ''
    try:
        isinstance(int(arg_value), int)
        value = int(arg_value)
        if 'threshold' in arg_name:
            if value < 0 or value > 100:
                output = str(arg_name) + ': the value must be between 0 and 100; '
        if 'indicator_expiration' in arg_name:
            if value < 0:
                output = str(arg_name) + ': the value must be positive (>0); '
    except Exception:
        return str(arg_name) + ': bad format, must be a number; '
    return output