`COAP_ACK_VARIANCE` is a configurable macro, so when it is 0, the [precondition for `random_uint32_range()`][1] is not held. [1]: https://doc.riot-os.org/group__sys__random.html#gab6ee09e1e56df3cc78acd1fbf97bfb24
`COAP_ACK_VARIANCE` is a configurable macro, so when it is 0, the [precondition for `random_uint32_range()`][1] is not held. [1]: https://doc.riot-os.org/group__sys__random.html#gab6ee09e1e56df3cc78acd1fbf97bfb24