Li ion conducting garnets have been attracting considerable interest for use as the electrolyte in all solid-state batteries, due to their high ionic conductivity and wide electrochemical stability window. Consequently, there have been a number of doping studies aimed at optimising the conductivity, focusing on both doping in Li7La3Zr2O12 and Li5La3(Nb/Ta)2O12 systems. In this paper, we report a detailed study of Pr doping in Li5La3Nb2O12, and show that this is a rare example of an ambi-site dopant, being able to be doped onto either the La or Nb site. Interestingly the resultant Pr oxidation state is determined by the site substitution, with oxidation states of 3+ for the La site, and 4+ for the Nb site. While the conductivity is essentially unchanged for the La site substitution, Pr4+ substitution on the Nb site leads to a large increase in the conductivity associated with the increase in Li content (Li5+xLa3Nb2-xPrxO12) up to 0.56 mScm-1 (at 50˚C) for x=0.8. Overall, this work highlights the flexibility of these garnet materials to doping, and suggests that further consideration of site substitution be considered for other dopants.