俱舍論疏 3.137

3.137

論「無苦根者至不善法故」，釋無苦所以。身淨妙故，無等流苦；無不善法故，無異熟苦也。

The commentary states: "Those lacking the faculty of suffering [...] unwholesome phenomena." This explains the reason for lacking suffering: because their bodies are pure and exquisite, they lack outflow suffering; because they have no unwholesome phenomena, they lack maturation suffering.