Maps in SSRS 2008

Yesterday I was getting an error while trying to run a report that contains a map, saying something like this: [...]maps binding or grouping for layer is not valid[...]. I don't remember exactly the entire error message, but I can reproduce it if one is interested.

This was a little tricky because I started to look at the map layer properties, trying to understand what I did wrong in the RDL. Just to find out that the DataSource was returning more than one row per country, so the poor RS didn't know which row to map.