String Trimmers

I recently purchased string trimmer SST25C. The assembly instructions for the shaft assembly indicated that the lower shaft is to be fully seated in the upper shaft and that the shafts will lock into place. I can insert the bottom shaft into the upper shaft, however, it will not insert far enough for it to lock in place. Are the two *** on the lower shaft supposed to be inside the upper shaft for it to lock in place? I am afraid to attempt to start the trimmer without knowing that the shafts are firmly in place.

Please let me know if this is the way the shafts fit together or if I need to return the product.

Thank You,
JKF