In the Berkeley RL class [CS294-112 Fa18 9/5/18](https://www.youtube.com/watch?v=XGmd3wcyDg8&feature=youtu.be&t=1071), they mention the following gradient would be 0 if the policy is deterministic.

$$
\nabla\_{\theta} J(\theta)=E\_{\tau \sim \pi\_{\theta}(\tau)}\left[\left(\sum\_{t=1}^{T} \nabla\_{\theta} \log \pi\_{\theta}\left(\mathbf{a}\_{t} \mid \mathbf{s}\_{t}\right)\right)\left(\sum\_{t=1}^{T} r\left(\mathbf{s}\_{t}, \mathbf{a}\_{t}\right)\right)\right]
$$

Why is that?