Document creation error using on-prem Appian

I am writing a custom Appian smart-service which should create a text document but I'm having issues. I have posted a snippet of the code below.

Document keyDoc = new Document();
keyDoc.setName(fileName);
keyDoc.setDescription(description);
keyDoc.setExtension("txt");
keyDoc.setParent(folderID);

documentID = contentService.create(keyDoc, ContentConstants.UNIQUE_NONE);
keyDoc.setId(documentID);


// Create document using Appian ContentService
File keyFile = new File(contentService.getInternalFilename(documentID));

When I test the smart service I receive the following error:

C:\appian184\appian\_admin\accdocs1\4072\10000845.txt (The system cannot find the path specified)

In addition, when I try to download the newly created document object, nothing happens (i.e., it doesn't download the file). 

Any ideas why this might be happening?