Blake Shelton Wishes Gwen Stefani a Happy 50th Birthday: 'I Love You So Much It's Actually Stupid'

Blake Shelton is showering girlfriend Gwen Stefani with nothing but love on her milestone 50th birthday.

On Thursday, the country star paid tribute to the No Doubt singer with a sweet message on Twitter. “Happy Birthday @gwenstefani!!!! I love you so much it’s actually stupid… And possibly illegal in the state of California,” Shelton wrote.

Next month, Shelton, 43, and Stefani will celebrate another milestone: their four-year anniversary of dating. The pair first met on the set of The Voice in 2014, and seven months later — after both of their respective divorces (Shelton from fellow country star Miranda Lambert and Stefani from Gavin Rossdale) — they sparked up a romance.

“It’s actually shocking that it has already been that long,” Shelton told PEOPLE in June of their upcoming anniversary. “It’s kind of a blur. It still feels like it’s pretty new to me. I guess it is, relatively. Four years isn’t forever, but man, it seems like it just happened in no time.”

RELATED: Blake Shelton Marvels at Celebrating 4 Years with Gwen Stefani This Fall: ‘It’s Kind of a Blur!’

In celebration of Shelton’s birthday in June, Stefani threw him an epic pizza party at her house and gifted him a huge custom welcome sign to put on display at his Oklahoma ranch. Additionally, Stefani’s sons Kingston, 13, Zuma, 10, and Apollo, 5 (whom she shares with Rossdale, 53) also wrote out birthday cards for him.

“Christmas, birthdays, any kind of holiday — they always sit down and take time to write a message and draw pictures or something,” he said. “Those are the coolest, coolest things, you know? It’s something I never experienced in my life. When they take time and actually make something, it’s pretty cool.”

Ahead of his birthday, Stefani also shared a tribute to Shelton on Instagram, affectionately calling him her “favorite human.”

“Can’t believe [you’re] mine ❤️🤷‍♀️ #soooooolucky!!!!” she wrote alongside a slideshow of throwback and more current photos of Shelton, including several shots of them cuddling up together.

The pair are currently co-workers on The Voice once again, this time joined by fellow coaches Kelly Clarkson and reigning victor John Legend for season 17. Shelton has appeared as a coach on every season of the NBC singing-competition series since its inception in 2011. Stefani, who first appeared on the show in late 2014 for season 7 and went on to coach seasons 9 and 12, was asked to return to the show after longtime coach Adam Levine announced his exit in May.

Last month, Shelton told PEOPLE that having Stefani back on set has helped with “the weirdness” of Levine, 40, not being there.

“He’s never not been here, you know?” he said. “And it’s just that guy that’s just over there constantly just makin’ you want to strangle him. And now he’s gone and it sucks, to be honest with you. But havin’ Gwen back, for me, makes it a lot more fun.”

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As to whether he and Stefani tend to take their work home with them, Shelton said, “It’s impossible to not.”

“We’re on The Voice together, so obviously, if something crazy happened, we’re still going to be talking about it when we get home,” he said. “And a lot of pillow talk about our artists and stuff.”

According to Shelton, Stefani is “taking 100 percent advantage” of the fact that he has a soft spot for her this season. For example, Clarkson, 37, noted Stefani has said, “Blakey, I want this one,” on the show when going head-to-head with him for an artist they both want on their teams.

“I’m going to fight for people that I think are great,” Stefani said. “If that means having to fight Blake hard, I’m gonna’ do it.”

Stefani went on to say that every season of she has worked with Shelton have been “completely different because I’ve been in different places in my life.” But, “this time’s my favorite because we know each other so well,” she said.

Along with her work on The Voice, Stefani will continue performing her Las Vegas residency shows at the Zappos Theater through Nov. 2.