Because it DOES work! :-)

I finally found this link, which helped me: http://stackoverflow.com/questions/9186983/non-parameter-sql-variables-with-mysql-connector-net-and-dapper

Conclusion: enabled it on the connectionstring and don't provide Dapper with a property to map.