Q: Does Lancer's Gáe Bolg not have any effect on anything without a heart, like certain creatures and machines? <Madao Gingetsu>

Takeuchi: It's you again.
Nasu: Placing a karmic reversal curse on something without a heart would not work. But you can definitely still use it for stabbing, physics permitting.
Takeuchi: Oh, I see. So like coming in clean from behind and destroying their central cavity.

Fate/Stay Night Animation Material I; Q&A, pg. 42