BaseCamp error in Position Format

There's an error in the window where you input a custom position format. The value I need to input is not allowed.
The same custom format works perfectly on handheld units (etrex legend, 60csx, 62st) and used to work in mapsource.

The figures match the Yacaré format, used in Uruguay (W55º48', +1, +500000, +4002288.2). But basecamp doesn't accept this last figure.

Is there a way to fix this or a workaround?

Thanks