There isn't one.
This actually happens a LOT with Ontario due to MTO not designating provincial route numbers on international bridges (even ON 405 doesn't end at the border, although TM hasn't trimmed that one for some reason, probably because the interchange that used to be at the endpoint is no longer). That said, even if MTO was more interested in designating numbers to the border, I'm not sure that they would here, since it just ends.