cifs errors on build 378.55

I'm using an RT-AC3200 with build 378.55.
Mounting a cifs share always returns an invalid argument:

# mount -t cifs //192.168.1.161/share /cifs1
mount: mounting //192.168.1.161/share on /cifs1 failed: Invalid argument

# mount -t cifs \\\\192.168.1.161\\share /cifs1
mount: mounting \\192.168.1.161\share on /cifs1 failed: Invalid argument

Searching the web, it seems this may be a problem with busybox. Has anyone else seen this issue?