After Amanda Bynes was arrested for allegedly throwing a bong out of her window, everyone assumed the cops would find copious amounts of drugs in her apartment and throw the book at her.

But it turns out they didn't -- so Amanda might be set to walk away from the incident without so much as a wrist-slap.

TMZ talked to an anonymous source in the NYPD who claims the cops have no hard evidence that Bynes was in possession of the chronic because none was recovered at the scene. Which, remarkably enough, actually backs up Amanda's story.

And despite police saying they smelled marijuana and saw her toss a bong out the window, they have nothing to show for it.

Days ago a TMZ producer went to inspect the subroof of Amanda's apartment building (where anything she tossed out the window of her 36th-floor pad would land), where he found some shattered glass remnants and tipped off the NYPD. Unfortunately for them, that glass -- which may or may not have been the bong she supposedly ditched -- sat outside for days.

Since there's no way to trace a chain of custody to Amanda, the evidence will probably be inadmissible in court. So even though early reports said she confessed to throwing what she calls a vase, it will be extremely arduous for prosecutors show evidence of whatever it was -- vase, bong or otherwise.

Meanwhile, Amanda maintains nothing was thrown at all in a new tweet, saying:

This is the first correct article except for one huge mistake! NOTHING was thrown out the window! I didn't throw anything out of the window! For them to say "she confessed to throwing a vase out of the window" is false and illegal!

So what she says about her case being dropped may actually turn out to be true. And we'll be treated to more episodes of the true-life Amanda train-wreck show for months to come.