ODataActionParameters is always null

I am using OData V4 and have an unbound method registered this way:

var getByHint = builder.Function("GetByHint");
getByHint.Parameter<string>("Hint");
getByHint.ReturnsCollectionFromEntitySet<SubCategory>("SubCategories");

This is the corresponding method in the controller:

[HttpPost]
[ODataRoute("GetByHint")]
public async Task<IHttpActionResult> GetByHint(ODataActionParameters parameters)
{
...
}

Then I send a POST request ( by using Fiddler):

http://localhost:57637/GetByHint

with the below headers:

Content-Type: application/json
Host: localhost:57637
Content-Length: 15

and the following body:

{"Hint":"test"}

The controller's method is being called but the parameters is always null, I enabled Web API tracing, this is the output:

iisexpress.exe Information: 0 : Request, Method=POST, Url=http://localhost:57637/GetByHint, Message='http://localhost:57637/GetByHint'
iisexpress.exe Information: 0 : Message='SubCategories', Operation=DefaultHttpControllerSelector.SelectController
'iisexpress.exe' (CLR v4.0.30319: /LM/W3SVC/2/ROOT-1-130844429306720269): Loaded 'C:\Windows\assembly\GAC_MSIL\Microsoft.VisualStudio.Debugger.Runtime\14.0.0.0__b03f5f7f11d50a3a\Microsoft.VisualStudio.Debugger.Runtime.dll'. Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
iisexpress.exe Information: 0 : Message='WebService.Controllers.SubCategoriesController', Operation=DefaultHttpControllerActivator.Create
iisexpress.exe Information: 0 : Message='WebService.Controllers.SubCategoriesController', Operation=HttpControllerDescriptor.CreateController
iisexpress.exe Information: 0 : Message='Selected action 'GetByHint(ODataActionParameters parameters)'', Operation=ApiControllerActionSelector.SelectAction
iisexpress.exe Information: 0 : Message='Will use same 'FormUrlEncodedMediaTypeFormatter' formatter', Operation=FormUrlEncodedMediaTypeFormatter.GetPerRequestFormatterInstance
iisexpress.exe Information: 0 : Message='Will use same 'JQueryMvcFormUrlEncodedFormatter' formatter', Operation=JQueryMvcFormUrlEncodedFormatter.GetPerRequestFormatterInstance
iisexpress.exe Information: 0 : Message='Model state is invalid. parameters: ', Operation=NonValidatingParameterBinding.ExecuteBindingAsync
iisexpress.exe Information: 0 : Message='Model state is invalid. parameters: ', Operation=HttpActionBinding.ExecuteBindingAsync

Can you help me figure out why parameters is null?