A locking unit of a camshaft adjuster (1) for an internal combustion engine is provided, in which a locking pin (19) establishes a positive connection between a driving element and a driven element of the camshaft adjuster (1). The driving element and/or the driven element are formed by a plastic part. An insert (26) against which the locking pin (19) rests and which is supported relative to the plastic part in the area of an inner and outer contour is inserted into the plastic part. Providing large area support of the contours makes it possible to use plastic as a material in spite of the great stress that occurs, particular attention being paid to a good introduction of force into the plastic part.