Better Pushback fails on sloped apron

[martinlest]

If your parking position is slightly sloped, when you release the parking brake for BP to begin its pushback, the plane immediately starts to slide forwards/backwards. Sometimes BP manages to 'grab' it before it has gone too far, other times it doesn't.. Any way to get BP to hold the a/c from the moment the request to release parking brakes is given? (Or some other fix for this?). Thanks.