Bad Mask /25 for address 198.133.234.3

Am I having a brain fart here? I am trying to reconfigure a 1001-X router running IOS-XE and getting the above message. I clear out the old IP address of an interface and enter:

ip address 198.133.234.3 255.255.255.128

It gives me: Bad Mask /25 for address 198.133.234.3

There are no other interfaces in this IP space and this isn't a network or broadcast address. I tried other addresses in the last octet as well.

It will accept 255.255.255.0. It will also accept ip address 198.133.234.129 255.255.255.128. But if I use any IP in the first half of that /24, it doesn't like it.

I tried configuring this on a switch with IOS-XE and it accepted it. So that points to either something in the configuration or an IOS bug.

Because of that I figured that IP space was conflicting with something else in the config. So I cleared out any mention of any IP address beginning with 198.133 in the config but still no dice. So it doesn't seem to be a conflict. Of course a conflict wouldn't explain why it would accept /24 and not /25.

I tried configuring the same IP on a clean, unused interface on the same router. Same thing. /25 in a different IP space works.

It won't accept anything smaller such as /26, /27, etc. either. It will only accept a /24.

So, why would IOS accept only /24 for a particular IP space in an interface config?