Forum:Magician's Circle and Mirror Force Question

Here's the scenario:

Player A has Strategist of the Ice Barrier on the field in attack position and has Magician's Circle set in the back row. Player B has a Mirror Force set in his back row.

(1) Player A attacks with Strategist and passes his priority. (2) Player B activates Mirror Force. (3) Player A activates Magician's Circle to chain Player B's Mirror Force. (4) Player B passes. (5) Magician's Circle resolves and Player A special summons Dai-sojo of the Ice Barrier from deck. Player B doesn't have spellcasters in his deck, so he didn't summon anything (Then Player A checks Player B's deck to verify that he really has no spellcasters). (6) Mirror Force resolves, but Player A's Ice Barriers are now protected by Dai-sojo's effect. (7) Strategist's attack goes through.

Which move(s) are breaking the rules?