An apparatus for sealing a cover film to a blister film having blisters holding respective small objects has a feed roll rotatable about an axis and formed with an array of radially outwardly open recesses arrayed like the blisters of the blister film. The blister film passes over the feed roll with the blisters in the recesses. A sealing roll is rotatable about an axis generally parallel to the feed-roll axis and presses the cover foil down against the blister foil on the feed roll. The feed roll is rotated to pull the films in a travel direction between the rolls so that the cover film is sealed to the blister film between the blisters thereof. A sensor detects movement of the sealing roll transversely of the feed-roll axis. A controller generates an output when the detected movement exceeds a threshold.