One is not allowed to mention the name of an idol and certainly not to swear an oath by an idol. Now for a regular prohibition of the Torah one gets lashes. [That is the punishment unless something else is stated openly.] But here things are a little different. One who swears by an idol has not done an act. The Gemara in Sanhedrin 63 side A says the opinion that one gets lashes for swearing by an idol is coming like R Yehuda who holds one can get lashes even for a prohibition that does not have an act. 

The Rambam brings that one does get lashes for this and the Raavad asks that that is only like R. Yehuda. But the law is not like R. Yehuda but rather R Yose who held one does not get lashes for a prohibition that does not have an act. 

Rav Shach answers that Rabin who stated the original version of the law before the Gemara made its corrections to his statement said simply one does not get lashes for a prohibition that does not have an act except swearing and vowing. [And that original statement was said according to the law which is like R Yose. And the way that original statement was said involved not just swearing a false oath but also swearing in the name of an idol.]

I am trying to figure out what possible reason the Rambam would have had to ignore the correction of the Gemara.