@RequestBody not parsing request into object for validation

I'm fairly new to Java/Spring and trying to set up an API Endpoint in an existing project. I've essentially copied some of the other endpoints that are currently working, but mine is not validating when being hit and it seems to be because the @RequestBody is not populating the object being fed into the method.

I've tried removing @NotNull but it's still failing out. This seems odd given that other endpoints are working w/the @NotNull.

SampleRequest.java

import NotNull;

public class SampleRequest {

    @NotNull
    private String testString;

    public void setTestString(String testString):
        this.testString = testString;

    public String getTestString():
        return testString;
}

SampleRequestResource.java

import Valid
import NotNull

public class SampleRequestResource {

    @NotNull;
    @Valid;
    private SampleRequest sample;

    public SampleRequest getSample():
        return sample;

    public void setSampleRequest(SampleRequest sample):
        this.sample = sample;
}

SampleController.java

import RequestBody
import RequestMapping
import RestController

@RestController
@RequestMapping("/foo")
public class SampleController(){

    @RequestMapping("/{id}/bar", method = request.POST, consumes = MediaType.APPLICATION_JSON_VALUE, produces = MediaType.APPLICATION_JSON_VALUE)
    public ResponseEntity<SampleResponseResource> stuff(
        @Valid @RequestBody SampleRequestResource request) {
            do stuff;
        return response;        
        })
}

test.py

import requests, json
header = {"Content-Type":"application/json"}
data = {"testString": "foo"}

test = requests.post(url, header=header, json=data, verify=false)

When I run test.py I expect it to return appropriately, however instead I get a validation error because sample is null from SampleRequestResource.java

I'm assuming the @RequestBody should parse the request and when it calls the SampleRequestResource it will push the parsed request to SampleRequest and validate just fine, as this is what seems to be happening in the rest of the API (like I said, I directly copied/altered other working endpoints to create mine.)