Please forgive my lack of detail here, it's been awhile since I ran across this bug. But, I believe that when creating a custom tech tree, the game breaks the link to a required foundation tech on the first tech in the list. Go back into your editor, and check the first tech (alphabetically listed, I believe) and relink it to the correct pre-requisite tech and you should be ok.
I may have the exact details wrong, and a forum search could probably turn up the solution. However, this is a known bug and it has been around for quite awhile.
Hope this helps...