Does a Summoner with a mithral shield suffer Spell Failure?

[Tharkon]

Summoner wrote:

A summoner can cast summoner spells while wearing light armor without incurring the normal arcane spell failure chance. Like any other arcane spellcaster, a summoner wearing medium or heavy armor, or using a shield, incurs a chance of arcane spell failure if the spell in question has a somatic component.

Magus wrote:

He can cast magus spells while wearing light armor without incurring the normal arcane spell failure chance. Like any other arcane spellcaster, a magus wearing medium armor, heavy armor, or a shield incurs a chance of arcane spell failure if the spell in question has a somatic component.

Does a Magus or Summoner incur arcane spell failure when wearing a chain shirt combined with a light mithral shield or a mithral buckler?

The way I see it there are two ways to read it.

A. The Summoner does not get ASF for light armor and the shield has 0% ASF for a total ASF of 0%.

B. The Summoner is using a shield and therefor incurse ASF, the total ASF is 20% from the armor + 0% from the shield, for a total of 20%.

Which way is the correct way to read it?

[zza ni]

hmm? there is no fail chance, the shiled has no fail chance. and the casters in case have an ability that treat ANY fail check from light armor to be ignored. the armor fail check is the armor fail check. it stack with a shiled but it still is the ARMOR's failed check. if you ignore it normaly, you still ignore it when it stack. so 0+0=0.

say you hve a dr 5/slashing and fire resitance 5 and are hit with a flaming piercing weapon for 4 piercce and 4 fire damage .the dr ignroe the first damage the resistance the 2nd and you take 0 daamge. you don't stack them to ignroe any part of the defences.