mysqli::real_escape_string() warning raised when resetting course and checklist progress

When resetting a course, if I check the checkbox to reset the checklist progress and user items, the following warning is displayed for, I think, each item the checlkist contains :

Warning: mysqli::real_escape_string() expects parameter 1 to be string, object given in [my_moodle_instal_dir]/lib/dml/mysqli_native_moodle_database.php on line 695

Status of the tasklist component is nevertheless "Ok" in the summary grid. If I click the "Continue" button to reset the course, no error is thrown but the progress of each user is still kept.