When Lindsay Lohan was sentenced to 90-days of lockdown rehab in New York as punishment for lying to the cops, it was assumed that lockdown rehab, you know, exists.

But as it turns out? Eh, not so much.

Since rehab facilities in New York state don't have lockdown rehab, intrepid news site TMZ decided to make some calls around the United States to see if LiLo could do her rehab elsewhere -- but no dice.

As Dr. Drew (who doesn't have the best track record) so helpfully explained, "There is no rehab that will hold you against your will, unless they feel the person is a suicide risk, and in that case they can hold the individual for 72 hours."

There is another rather ironic exception: New York does have a state-run lockdown rehab facility, but the only way anyone gets into that is by being a convicted felon. And while Lindsay has been in court more times than your average felon, she doesn't have any actual felony convictions.

What we're saying is she's not eligible to serve out the terms of her sentence in the one place it's actually possible -- because her record is too clean. Which is actually kind of hilarious.

And why didn't prosecutors look into all this before they accepted the word of Lohan's attorney Mark Heller, a man who carries a lucky rabbit's foot on his Louis Vuitton briefcase? No telling, but insiders say authorities are currently re-evaluating La Lohan's plea deal -- and jail is now back on the table.