fsck-object --standalone got errors

I have a repository created by GIT itself(not cognito, etc). It has 
branches called base, master and origin. When I did git-fsck-objects 
--full there is nothing shown, but when I did git-fsck-objects 
--standalone, it displayed following:

error: refs/heads/master: invalid sha1 pointer 
ea51c414519ffe78c5bf95c488e94e82d3603472
error: refs/heads/base: invalid sha1 pointer 
4b75aaeb5af2dc69374ad080020758e4de6a45d2
error: refs/heads/origin: invalid sha1 pointer 
f4c9503abb2ad52752004e8e9b77b72e23d18d3e
fatal: No default references


The question is:
what's the purpose for this standalone check? what's these errors about? 
Do I need to care these errors?
what's the step to ensure my repository always be in a good shape? Is 
running fsck-objects --full sufficient?


Thanks
Ming