Problem with mine fields

Ok. Here is what happens. I sweep in and clear a planet of its defensive units, except that it doesnt have its minefield deployed yet. So my fleet occupies the planet, and the mine field is not yet deployed. Next turn, the planet deploys its mine field, with my ships in orbit. I have a mine-sweeper ship, but it cannot remove the mines... I use the "B" option, and nothing happens. Any suggestions?