Box API returns HTTP 403 when trying to collaborate APP-USER

I am having a problem trying to give collaboration rights to an App-User. 

First, I create an enterprise API connection.  Works fine.

Second, I create App User,

Third, I try to give App User access rights to the root folder of the owner:

BoxFolder root = BoxFolder.getRootFolder(enterpriseAccess);
root.collaborate(userInfo.getLogin(), BoxCollaboration.Role.VIEWER_UPLOADER);

The collaborate method takes BoxAPIException with message:  "The API returned an error code: 403"

Here is my Java code to create a Box App-user connection:

BoxAPIConnection enterpriseAccess = BoxDeveloperEditionAPIConnection.getAppEnterpriseConnection(boxConfig.getEnterpriseId(), 
																													boxConfig.getClientId(),
																													boxConfig.getClientSecret(),
																													encryptionPreferences,
																													accessTokenCache);					                                                                     
					if (proxy != null) {						
						enterpriseAccess.setProxy(proxy);
					}
					CreateUserParams cup = new CreateUserParams();
					cup.setIsPlatformAccessOnly(true);
					BoxUser.Info userInfo = BoxUser.createAppUser(enterpriseAccess, APP_USER_NAME, cup);				
					//
					//	Now we need to give App User collaboration rights:
					//
					BoxFolder root = BoxFolder.getRootFolder(enterpriseAccess);
					root.collaborate(userInfo.getLogin(), BoxCollaboration.Role.VIEWER_UPLOADER);
					//
					this.boxAppUserConx = BoxDeveloperEditionAPIConnection.getAppUserConnection(userInfo.getLogin(),
							                                                    				boxConfig.getClientId(),
							                                                    				boxConfig.getClientSecret(),
							                                                    				encryptionPreferences,
							                                                    				accessTokenCache);					
					//
					this.boxAppUserConx.setRequestInterceptor(new RequestInterceptor() {
						/**
						 * @see com.box.sdk.RequestInterceptor#onRequest(com.box.sdk.BoxAPIRequest)
						 */
						public BoxAPIResponse onRequest(BoxAPIRequest request) {
					        request.addHeader("As-User", boxConfig.getAppUserName());
					        // Returning null means the request will be sent along with our new header.
							return null;
						}
					});