As a general rule, if a unit is both prevented from making, and guaranteed to make a follow-up attack, the effects cancel out, and the Spd difference decides instead (including effects that increase the Spd difference such as Wyvern Rift).
A foe with effects such as Phys. Null Follow or Null Follow-Up will neutralize the guaranteed follow-up attack granted by Follow-Up Ring.
Wyvern Rift).
Phys. Null Follow or 
Null Follow-Up will neutralize the guaranteed follow-up attack granted by Follow-Up Ring.
