The diplomatic thing to do would be to say the new film of “Harold and the Purple Crayon” is misguided, dull and mostly humorless and leave it at that.

It’s harmless enough, which is the bare minimum parents expect from a kids movie.

And it’s no easy thing, taking a slight but warm children’s picture book and making a feature film out of it. A short film, sure. But that came out in 1959, four years after Crockett Johnson’s book about a child inventing adventures for himself by drawing his way into odd situations with strange creatures thanks to his “magical” purple crayon. There was also an animated TV series aimed at the very young back in the early 2000s.

But almost from the moment the story of Harold stops being animated in the prologue, his life narrated (by Alfred Molina), this children’s entertainment goes wrong.

It parks our now adult hero (Zachary Levy) and his besties Moose (Lil Rel Howery) and Porcupine (Tanya Reynolds) in the “real world,” and that fish-out-of-water joke wears out before the punchline.

With an “imaginary friend” element, it’s similar enough to the early summer blockbuster “IF” to beg comparison, but this puts the problematic star of “Shazam” and other childish adults from a book — they don’t realize that, at first — in an adult world instead dragging imaginary friends into adult life with “Deadpool”/Ryan Reynolds.

While “Harold” never ceases to be childish, “entertainment” almost never enters the picture.

Harold tires of the simple lines and scenarios of the printed page and draws himself into reality and pops up in Providence, Rhode Island in an adult-sized onesie. The “kind and wise old man” is no longer narrating his story and telling him what to do, so he’s at a loss.

The fact that Moose tumbles into the real world with him is no help. Porcupine makes the transition later, showing up with a British accent and Goth girly wardrobe, sniffing around for Moose and Harold and getting into trouble for it. .

After drawing them a purple tandem bicycle to explore their new world with, Harold and Moose are hit by a Subaru driven by a widowed mom (Zooey Deschanel), with her imaginary-friend-obsessed pre-tween Mel (Benjamin Bottani).

Mel learns of Harold’s crayon super power when the dorky adult magically repaints Mom’s house, draws a kitchen full of berry pies and draws a purple plane to fly off in as they search for “The Old Man” narrator.

Harold, Moose and Mel run afoul of Gary the Librarian (Jemaine Clement), a frustrated fantasy novelist who figures out that “Harold” and his “Purple Crayon” come from a book published during the Eisenhower administration.

Gary covets that crayon, making him the villain.

Mel has enough “pure imagination” to be gifted with half his crayon by Harold. That just adds to all the trouble bullied Mel gets into while not lessening Harold’s capacity for blundering mischief.

The almost-funny bit of the movie has Harold and Moose fill in on Mom’s job, in the stockroom of the discount chain Ollie’s. Anybody who’s ever been in an Ollie’s will feel cheated that their Ollie’s — typically an oversized and even more cluttered Dollar Tree — doesn’t look more like the swank down-market Macy’s depicted here.

Deschanel has nothing to play, save for the purple piano Harold draws her to get her back into music.

Clement is in rare form, unable to land a single laugh thanks to this David Guion/Michael Handelman script. They count a “Night at the Museum” sequel and the far more inventive but just as clunky “Slumberland” among their duo-credits.

Howery puts in a lot of effort in pursuit of laughs, to little avail.

And Levy, while relieved to get the work and still willing to tap into his inner child, adds nothing light, insightful or amusing to a children’s movie that was stillborn from the start.

Maybe if he’d kept wearing that onesie.

Rating: PG

Cast: Zachary Levy, Zooey Deschanel, Lil Rel Howery, Benjamin Bottani, Tanya Reynolds and Jemaine Clement, narrated by Alfred Molina

Credits: Directed by Carlos Saldanha, scripted by David Guion and Michael Handelman, based on the children’s picture by Crockett Johnson. A Columbia/Sony release.

Running time: 1:32