Problems with GoogleCode SVN

I use GoogleCode (http://code.google.com) for my little open source projects, and it usually works quite nicely.
I was just trying to set up one of my repositories in writable mode on a new computer, and "svn ci" (checkin) gave 500-Internal Server errors, which was not very helpful.
Eventually I realised that although I had done the "svn checkout" with a --username=", when I was doing the checking it was using my computer username, rather than my google one...so presumably the authentication failed.
I had to do "svn ci --username=", then it worked ok.   I don't know why!