Heyo,  

We have a document object with property `Id` (that we know auto maps to the `_id` field) and we can get it to work as an `int` and a `System.Guid`. 

However when we change it to a string, which would better fit our use case, we get the following exception when trying to index:

`mapper_parsing_exception Reason: failed to parse [id] CausedBy: Type: illegal_argument_exception Reason: For input string: "R700"`

Not really sure what to try here. We want to have a string `_id` and according to online resources we should be able to, however we get this exception.
Any help much appreciated!

**EDIT**: we are inferring the Id from a property named Id on the POCO, I should have clarified.
Example:

```
public class MyDocument {
    public string Id { get; set; }
    ...
}

```