Dropbox API - files/list_folder : request body: could not decode input as JSON

I have a windows app that creates an HTTP request.  When I use apis that don't have arguments, everything works fine, but when I use one that needs arguments in the body like the files/list_folder, I can never get it to work.  I'm definitely putting json in there, but for some reason it doesn't recognize.  I've tried it as a string and also blob, with and without UTF8 encoding and always get the same error.  Here is my ful request as trapped from Fiddler:

POST https://api.dropboxapi.com/2/files/list_folder HTTP/1.1
Connection: Keep-Alive
Content-Type: application/json
Authorization: Bearer mytoken
User-Agent: api-explorer-client
Content-Length: 14
Host: api.dropboxapi.com

{"path":"/mysharedfolder"}

I have also tried it with no folder just "" and same error.