Lindsay Lohan -- Taxes Just Ain't My Bag, Baby

Lindsay Lohan had no idea she was so deeply in the hole to Uncle Sam ... because she THOUGHT she had accountants to take care of that crap for her ... sources close to Lindsay tell TMZ.

TMZ broke the story ... the I.R.S. hit Lohan with a big, fat tax lien, claiming she failed to fork over $93k in federal income taxes from 2009.

Now, sources close to Lindsay tell TMZ she was completely unaware of the problem. We're told Lindsay actually cleaned house recently and replaced most of her business team.

Our sources say Lindsay believes the debt was most likely lost in the shuffle, but the issue will be handled immediately.