Objective C batchUploadFiles gives error: "missing required field 'session_id'" or "Garbage at end."

I'm actually receiving the same error, but I've got the correct code in the response block (I think).  It doesn't seem to happen on the first batch upload call during a run, but the second upload attempt seems to fail pretty consistently.  Here's my code, if it helps.

[self.fileRoutes batchUploadFiles:filesToUpload
                                queue:self.fileUploadQueue
                        progressBlock:^(int64_t uploaded, int64_t uploadedTotal, int64_t expectedToUploadTotal) {
                            if (fileSize.intValue > 0) {
                                float progress = (float) uploadedTotal / fileSize.longLongValue;
                                photo.uploadProgress = progress;
                            }
                        }
                        responseBlock:^(NSDictionary<NSURL *, DBFILESUploadSessionFinishBatchResultEntry *> *fileUrlsToBatchResultEntries,
                                DBASYNCPollError *finishBatchRouteError, DBRequestError *finishBatchRequestError,
                                NSDictionary<NSURL *, DBRequestError *> *fileUrlsToRequestErrors) {
                            DBFILESUploadSessionFinishBatchResultEntry *result = fileUrlsToBatchResultEntries[srcPath];
                            if (result.isSuccess) {
                                [self uploadedFile:destPath from:srcPath.absoluteString metadata:result.success];
                            } else {
                                NSMutableDictionary *userInfo = [[NSMutableDictionary new] autorelease];
                                userInfo[@"path"] = srcPath.absoluteString;

                                if (fileUrlsToRequestErrors) {
                                    NSMutableDictionary<NSURL *, NSString *> *errors = [[NSMutableDictionary new] autorelease];
                                    for (NSURL *key in fileUrlsToRequestErrors.allKeys) {
                                        errors[key] = fileUrlsToRequestErrors[key].errorContent;
                                    }
                                    userInfo[@"fileErrors"] = errors;
                                }

                                if (finishBatchRouteError) {
                                    userInfo[@"routeError"] = finishBatchRouteError.description;
                                }

                                if (finishBatchRequestError) {
                                    userInfo[@"requestError"] = finishBatchRequestError.errorContent;
                                }

                                // TODO Handle conflicts here
                                if (result.failure.isPath){
                                    if (result.failure.path.isConflict) {
                                        // TODO Handle This
                                        DLog(@"Conflicting filename: %@, %@", photo.filename, result.failure.path.conflict)
                                    }
                                }

                                NSError *err = [NSError errorWithDomain:@"my.app" code:0 userInfo:userInfo];
                                [self uploadFileFailedWithError:err];
                            }
                        }];