Historical Rules · 27 September 2013
Rule 603.3d
Official source text from the 27 September 2013 Rules release
The remainder of the process for putting a triggered ability on the stack is identical to the process for casting a spell listed in rules 601.2c-d. If a choice is required when the triggered ability goes on the stack but no legal choices can be made for it, or if a rule or a continuous effect otherwise makes the ability illegal, the ability is simply removed from the stack.