Class 'completion_info' not found

A PDF file resource is tagged with Activity completion as follows:
Completion tracking "Show actiivty as complete when conditions are met"
and
Require view is ticked "Student must view this activity to complete it"

The next resource is hidden until the student views the PDF file. Once the student has viewed the PDF file, the webpage resource (created via selecting the "Page" option in the "Add a resource" drop down box) appears. When the student clicks on it, s/he gets an HTTP 500 error. The corresponding server error log entries is:

PHP Fatal error: Class 'completion_info' not found in /.../moodle/lib/conditionlib.php on line 391, referer: http://../moodle/course/view.php?id=2

This webpage has the following Restrict availability conditions:

Activity completion condition: PDF File "must be marked complete"
Before activity is available: "Hide activity entirely"

And the Activity completion is set as follows:
Completion tracking "Show actiivty as complete when conditions are met"
and
Require view is ticked "Student must view this activity to complete it"