Paulina Rubio's ex-husband wants her to undergo a psychiatric evaluation.

The 'X Factor' USA judge, 42, is embroiled in a bitter custody battle with her former spouse, Nicolas Vallejo, whom she divorced last year after almost five years of marriage, and he has asked a judge to force the Latin singer to prove she is a mentally fit to care for their son, Andrea Nicolas, who will celebrate his 3rd birthday in November.

A source told gossip website TMZ.com that Nicolas is pulling out all the stops to make the Mexican singer complete the test, as previously ordered by the judge, and is kicking up a fuss because he wants the judge to make it clear that it is compulsory.

The former couple have been fighting for custody of their son for more than a year, and have both accused each other of alienating Andrea from them.

Nicolas also claims that Paulina took over caring for the little boy on August 3 and he hasn't seen Andrea since then.

The blonde beauty is currently living in Los Angeles to film the Fox reality TV series alongside Simon Cowell, Demi Lovato and Kelly Rowland.

Paulina has reportedly been dating her fellow Latin American singer Gerardo Bazua, 28, since May.