I believe this has been discussed before on this board, but I was unable to find the thread.
Recently, a rogue IMC used sneak attack on a creature with damage reduction. His rapier was unable to penetrate it, so I ruled that the sneak attack damage was negated as well. This was not a popular ruling, especially since I was unable to find a clear rule in the PHB or DMG to back it up. The rogue felt that sneak attack is different from poison or energy damage, and should take effect even when the damage isn't high enough.
Is there a rule that covers this specifically, or is it open to interpretation? What have some other DMs done about this kind of situation?
Recently, a rogue IMC used sneak attack on a creature with damage reduction. His rapier was unable to penetrate it, so I ruled that the sneak attack damage was negated as well. This was not a popular ruling, especially since I was unable to find a clear rule in the PHB or DMG to back it up. The rogue felt that sneak attack is different from poison or energy damage, and should take effect even when the damage isn't high enough.
Is there a rule that covers this specifically, or is it open to interpretation? What have some other DMs done about this kind of situation?