Create and insert new document into specific folder via (sfdc) REST API?

Question

In order to create a new document and insert it to a specific folder via API, must I specify the folder in Database.insert (doc); method?

I am getting a missing required fields error when testing saying I am missing FolderId and Name fields but they are listed, my method is as follows:

Document doc = new Document();

doc.FolderId = (string)requestJson.get('SFDC unique ID of targeted folder");

doc.Body = Blob.valueOf('JSON string');

doc.Id = (string)requestJson.get('Id');

doc.Name = (string)requestJson.get('Name');

Database.insert(doc);

Map<String, Object> m = new Map<String, Object>();
m.put('success', true);
response.responseBody = Blob.valueOf(JSON.serialize(m));