At the bottom of 61a, the Gemoro says that the Torah specifies an Issur of Gezela to create an extra Lav for Kovesh S'char Sochir.
Tosfos says that an extra Lav would not be used for for Gezela itself, since Gezela is "Nitak LeAseh" (hence no Malkus), whereas Kovesh S'char Sochir is not.
My question is that Kovesh S'char Sochir is presumably a Lav SheEin Bo Maaseh, so there would not be Malkus for it either?!
The Chacham Tzvi (#26) asks this question on Tosfos and remains with a "Tzarich Iyun." The Noda b'Yehudah (Mahadura Tinyana OC 91) shows how this Sugya is consistent with the view of Reish Lakish who maintains that "Lav she'Ein Bo Ma'aseh Lokin Alav."