Command block minecart is not its own command sender

The bug

@s doesn't work with command block minecarts.

How to reproduce

Place a command block minecart on a powered activator rail. Add the command /say @s to it.

Expected Results

Repeatedly says "Command Block Minecart"

Observed Results

Repeatedly sends empty message

Notes

This is the case for all uses of @s, not just /say