If your dataset includes documents that may be either JSON or binary, then you do not want to create a view that outputs individual fields for non-JSON documents. You can fix this by using a view that checks the metadata type field before outputting the JSON view information:

function(doc,meta) { 
    if (meta.type == "json") { 
        emit(doc.firstname.toLowerCase(),null);
    }
}

In the above example, the emit() function will only be called on a valid JSON document. Non-JSON documents will be ignored and not included in the view output.