Custom field of type time cannot be sorted

[massimo.b@…]

In [ticket-custom] we created a due date field for tickets:

duedate = time
duedate.format = date
duedate.label = Due date
duedate.order = 14

Creating new tickets works and the field can be seen in custom queries, but pressing the column for sorting results in this error:

 Trac detected an internal error:

ProgrammingError: COALESCE types text and integer cannot be matched
LINE 5: ORDER BY COALESCE("duedate".value,0)=0,"duedate".value,t.id)...
                                          ^