Not A Bug forced to deal trample damage to player

i have a 5/3 deathtouch trample.. which had 4 blockers declared

my opponent had death triggers on 3 of those creatures i did not want to activate.

so i tried to assign 5 damage to to the first creature and do deal no damage to any other creature and no trample damage.

but the game decided that was illegal and didn't let me.

according to rule 702.19b i should be able to assign all damage to the first creature and do none to the other creatures or the player.

the death triggers of the other 3 creatures which died because the game forced me to deal damage to them killed me
 
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