And you didn't notice before handing in your assignment?
Doing things last second that didn't give you any chance to fix issues before a deadline is a flaw in and of itself and should be a lesson.
Unfortunately many don't understand this and I have many colleagues who slack off until it's almost too late and then there is a huge scramble before a deadline.
The non-PEBKAC problem was that I was using Turbo Pascal 2.0 (1.0?) that couldn't identify I'd screwed up a method's parameter definition.