Extruding wrong surface

Hi, I am going through the manual training level 1 exercise 42 page 122. It has been ok so far, but when extruding the curve of what looks like a mechanical element, it transforms into solids the holes instead of the main surface. Any ideas what I am doing wrong?

It means that the curves describing your outer boundary do not form a closed loop. Try to use the “connect curves” command to accurately close the corners.

Thanks a lot. There was a half a millimeter gap in my model.

CrvEnd and CrvStart are useful for locating the gaps or overlaps in curves that you think ought to be closed.