One simple rule for dating Bruce Willis’ 32-year-old daughter: Don’t! An OK! Insider claims that the Die Hard star, 65, was less than thrilled to find out that his eldest child, Rumer, went on a date with the newly single Armie Hammer.

Fueling Bruce’s papa-bear fire? The fact that the 34-year-old actor — still technically married to estranged wife Elizabeth Chambers, from whom he split in July — was seen courting Josh Lucas’ ex-wife, Jessica Ciencin Henriquez, just days before his outing with Rumer!

“Bruce doesn’t want his little girl to get hurt,” explains the insider. While the DWTS champ’s mom Demi Moore “is letting Rumer make her own choices,” notes the insider, Bruce is less likely to butt out.

RUMER HAS IT: ARMIE HAMMER MOVES ON WITH WILLIS BEFORE HIS DIVORCE IS FINALIZED

He’s very protective of all his daughters,” explains the insider, “but Rumer is his firstborn, and he thinks she should know better.”

In early July, Hammer and Chambers announced they had called it quits in a joint statement shared on their respective Instagram accounts.

“Thirteen years as best friends, soulmates, partners and then parents. It has been an incredible journey, but together, we’ve decided to turn the page and move on from our marriage,” the statement read. “As we enter into this next chapter, our children and relationship as co-parents and dear friends will remain our priority.

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“We understand this news lends itself to public dialogue, but in the interest of our children and our family, we’re asking for privacy, compassion and love during this time.”

Chambers filed for divorce from the Call Me By Your Name actor the same month. Chambers cited irreconcilable differences as the reason for their split. She also requested that the court grant her primary physical custody of their children, along with joint legal custody for herself and Hammer, as well as spousal support.

The former couple got married in May 2010 and share two beautiful children together: daughter Harper Grace, 5, and son Ford Douglas Armand, 3.

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