There is <A target="_blank" href="http://blog.nashcom.de/nashcomblog.nsf/dx/LS2CAPI-is-broken-in-64bit-d8.0.2-and-d8.5.htm">a known issue with calling C-API</A> from Lotus Script for Win64. Daniel Nashed warns if you want to develop C-API from Lotus Script on Domino Win64 you also need to get a hotfix when 8.5 ships in January.