I don't think that's an error, it's just a bit of archaic grammar. The modern word 'welcome' is a contraction of what used to be a phrase, 'well come'. The evolution of language usually trends toward simplification and shorter ways to get the same concept across. e.g. Can not => cannot => can't.

Stuff like 'lol' and replacing words like 'you' and 'are' with single letters are also examples.