Getting API access tokens with Golang

I am using Golang to work with the Box API. However, I am trying to generate access tokens through the method of using JWT to eliminate the need for user interaction beyond initial setup. However, after creating a JWT and trying to send it in an oAuth request, I am failing to retrieve an access token. The following to functions create a JWT to use in an oAuth request to gain an access token. However, the response body is empty and the HTTP request generates a 400 (Bad Request) error. After the functions I show the result of printing out the resp.Status and resp.Body

// JWTAUTHURL - URL for oAUTH for Box
const JWTAUTHURL string = "https://api.box.com/oauth2/token"

// JWTGRANTTYPE - mandatory parameter for box oAuth
const JWTGRANTTYPE string = "urn:ietf:params:oauth:grant-type:jwt-bearer"

// oAuthResponse holds decoded JSON response from Box
type oAuthResponse struct {
	AccessToken  string   `json:"access_token"`
	Expires      int      `json:"expires_in"`
	RestrictedTo []string `json:"restricted_to"`
	TokenType    string   `json:"token_type"`
}

// CreateJWTAssertion - build up the JSON Web Token for oAuth
func CreateJWTAssertion(PublicKeyID string, ClientID string, Sub string, user *AppUserResponse) (string, error) {

	var signingKey []byte
	var err error
	var msg, tokenString string

	signingKey, err = ioutil.ReadFile("private_key.pem")

	if err != nil {
		msg = "Unable to read signing key. Please ensure your private signing key is in the ./keys/ directory"
		return msg, err
	}

	// Generate JTI Value
	jti, err := exec.Command("uuidgen").Output()
	if err != nil {
		msg = "Unable to generate JTI value"
		return msg, err
	}	

	token := jwt.New(jwt.SigningMethodHS256)
	claims := make(jwt.MapClaims)
	token.Header["alg"] = "HS256"
	token.Header["typ"] = "JWT"
	token.Header["kid"] = PublicKeyID
	claims["iss"] = ClientID
	claims["aud"] = JWTAUTHURL
	claims["jti"] = jti
	claims["exp"] = time.Now().Add(time.Second * 45).Unix()

	// Create the JWT either for the enterprise or for a specific user.
	if user.ID != "" {
		claims["box_sub_type"] = "user"
		claims["sub"] = user.ID
	} else {
		claims["box_sub_type"] = "enterprise"
		claims["sub"] = Sub
	}
	token.Claims = claims

	// Sign the JWT
	tokenString, err = token.SignedString(signingKey)

	if err != nil {
		msg = "Unable to sign token, please check that you have a signing key in ./keys/"
		return msg, err
	}

	return tokenString, err
}

// SendOAuthRequest - Sends a POST to authenticate against Box using JWT Assertion
func SendOAuthRequest(ClientID string, ClientSecret string, JWToken string) (string, error) {

	var err error
	var msg string
	var decodedResponse *oAuthResponse

	hc := http.Client{}
	form := url.Values{}

	// Build form to POST
	form.Add("grant_type", JWTGRANTTYPE)
	form.Add("client_id", ClientID)
	form.Add("client_secret", ClientSecret)
	form.Add("assertion", JWToken)

	// Here is where the code stops working, returns an empty body
	// The request looks like it should be working
	req, err := http.NewRequest("POST", JWTAUTHURL, strings.NewReader(form.Encode()))
	req.PostForm = form
	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
	if err != nil {
		return msg, err
	}

	resp, err := hc.Do(req)
	if err != nil {
		msg = "Error submitting request to Box API"
		return msg, err
	}

	//
	fmt.Println(resp.Status)
	fmt.Println(resp.Body)
	//

	err = json.NewDecoder(resp.Body).Decode(&decodedResponse)
	
	if err != nil {
		msg = "Error decoding OAuthResponse"
	} else {
		// We only need the Access Token
		msg = decodedResponse.AccessToken
	}

	return msg, err
}

400 Bad Request
&{0xc420064100 {0 0} false  0x5f48b0 0x5f4840}