The second argument of external_format_value constructor should be integer - either VALUE_OPTIONAL, VALUE_DEFAULT or VALUE_REQUIRED.

In 2 cases in master it's mistakenly set as string (description):

mod/page/classes/external.php
191:                            'contentformat' => new external_format_value('content', 'Content format'),

tag/classes/external.php
250:                        'descriptionformat' => new external_format_value(PARAM_INT, 'tag description format'),