Passing a scalar to `map` should throw an InvalidArgumentException

When for example an array is passed to `map`, `get_class` will trigger an error. It would be better to check the type and throw an `InvalidArgumentException` if needed. In the future, we could add mappings from scalars (such as arrays, #18)

Start by locating the map implementation where get_class is called and inspect how invalid input is currently handled. Verify that passing a scalar produces InvalidArgumentException while existing valid mapping behavior remains unchanged.

Written by the indexing model from the issue text.