custom format in Numbers cell

I'm trying to create a custom number format in a Numbers spreadsheet. I create the format, apply it to a cell, then when I exit / move from the cell, the cell format reverts to "Automatic".

The format I'm trying to create is

1234-567

and I'm doing it like this

####-###

where the "-" is typed (I also tried the "Space" token).

Any cell formatted with this custom format reverts to "Automatic".

What am I doing wrong?

Thanks!