Error when trying to debug SP Designer 2013 workflow

I have installed the workflow manager, created a SP 2013 workflow and it runs just fine.

However, when I try to get 'WorkflowDescription' programmatically, I get an error which doesn't help me getting any further

Workflow is deployed in the list 'Document_Library' and I'm trying the code below:

WorkflowManagementClient client = new WorkflowManagementClient(new Uri("http://ServerName/sites/SiteName/Document_Library"));
Collection<WorkflowDescription> descriptions = client.Workflows.Get(0, 1);

When I try to debug this code I get an error:

A response was returned that did not come from the Workflow Manager. Status code = 404:
404 NOT FOUND Client ActivityId : 2609d79a-6dd7-4973-945a-67cebb4a9e24

Any suggestions how I can get past this error, or is there any better way to get workflow details?