Cisco router will not accept subnet mask

[ATC]

Cisco router refuses to set the correct subnet mask, complaining about "Bad mask /xx for address y.y.y.y".

Both the address and the subnet mask has been double-checked and is valid.

[ATC]

Enable "subnet-zero" under config like this:

"conf t"
"ip subnet-zero"
"end"
"wr"

You should now be able to set the subnet mask.