I have a table that contains these numbers in a column:

mysql> select distinct course_id from programme_courses;
+-----------+
| course_id |
+-----------+
| 1 |
| -1 |
| -2 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 65 |
| 66 |
| 67 |
| 68 |
+-----------+
14 rows in set (0.00 sec)

and I have these values in a different table:

mysql> select course_id from courses;
+-----------+
| course_id |
+-----------+
| 65 |
| 66 |
| 67 |
+-----------+
3 rows in set (0.00 sec)

I'm looking to find those values in the first query that aren't in the
second, so I tried this:

mysql> select distinct programme_courses.course_id from
programme_courses where course_id not in (select course_id from courses);
+-----------+
| course_id |
+-----------+
| 1 |
+-----------+
1 row in set (0.00 sec)

which somewhat puzzles me. If I replace the subquery with the values I
know it returns, it works as required:

mysql> select distinct programme_courses.course_id from
programme_courses where course_id not in (65,66,67);
+-----------+
| course_id |
+-----------+
| 1 |
| -1 |
| -2 |
| 2 |
| 3 |
| 4 |
| 5 |
| 6 |
| 7 |
| 8 |
| 68 |
+-----------+
11 rows in set (0.00 sec)

Can someone explain where I'm going wrong with my subquery?