Why cant I summon Zeren the chainbound dragon from the gy

[snow grotto]

Hi, So secreterion dragon’s effect states that I can target 1 dragon and 1 spellcaster in my gy, shuffle one and summon the other, however, even thought it allows me to target Zeren as a valid target, I can’t seem to be able to summon him. It doesn’t matter how many times I checked I can’t find any real reason why it doesn’t work. Can anybody explain?

[sage reef]

You cannot special summon monsters with a summoning condition (xyz, synchro, link, etc.) from a public location (GY, Banishment, S/T zone) if it was not properly summoned first.