"I just can't concentrate in flats," she told the U.K.'s GMTV on Thursday.
In fact, the pint-sized pop star's shoe preference is so pronounced, she can't possibly work out.
"I'd love to go to the gym," she told the morning chat fest, "but I can't get my head around the footwear."
There is, however, an exception to her strict no-flats policy. "When I'm at home with [husband David Beckham] and the kids, I'm always in a pair of flip flops," she admitted. "He likes me without any makeup on – just looking really natural."