What we always have to remember is that we seldom can estimate on a completely solid ground. That's why we normally introduce a corrective factor, which is absurdly big at the start of a project and narrows while the latter proceeds.
Sometimes you lack important pieces of information, nevertheless you have to guesstimate. It should be clear to everybody (at least it is crystal clear to me) that if you guesstimate an effort of 100 it could actually be a value between 60 or 160 (PMI says between 75 and 175, which is a more pessimistic but probably better correction). Guesstimates are not contracts, and they cannot (they shouldn't) be used against you. Thus, we must not be afraid of guesstimating; it often helps us to clarify some aspects of what's ahead of us.
"I don't have enough information" is a very good justification, and it must be respected, but it cannot be an excuse. I know it can be subtle, but that's how I see it. After all, as an old keyring of mine reported...
right or wrong, I'm still the captain