diff --git a/src/grader_interface.ts b/src/grader_interface.ts
index d38b2a0..4b517b1 100644
--- a/src/grader_interface.ts
+++ b/src/grader_interface.ts
@@ -166,10 +166,6 @@ WHERE tm_submissions.ID = :idSubmission
     throw new InvalidInputError('Cannot find source code associated with this submission');
   }
 
-  if (!submissionData.answerToken && !appConfig.testMode.enabled && 'UserTest' !== submission.sMode) {
-    throw new InvalidInputError('Missing answerToken, required for this type of submission');
-  }
-
   let tests: TaskTest[] = [];
   if ('UserTest' === submission.sMode) {
     tests = await Db.execute<TaskTest[]>('SELECT tm_tasks_tests.* FROM tm_tasks_tests WHERE idUser = :idUser and idPlatform = :idPlatform and idTask = :idTask and idSubmission = :idSubmission ORDER BY iRank ASC', {
diff --git a/src/platform_interface.ts b/src/platform_interface.ts
index 7c5969e..be2042f 100644
--- a/src/platform_interface.ts
+++ b/src/platform_interface.ts
@@ -49,7 +49,7 @@ export async function extractPlatformTaskTokenData(token: string|null|undefined,
     }
   }
 
-  if (!payload.idUser || (!payload.idItem && !payload.itemUrl)) {
+  if (undefined === payload.idUser || null === payload.idUser || (!payload.idItem && !payload.itemUrl)) {
     throw new InvalidInputError('Missing idUser or idItem in token');
   }
 
diff --git a/src/submissions.ts b/src/submissions.ts
index 6955371..1ba3a97 100644
--- a/src/submissions.ts
+++ b/src/submissions.ts
@@ -168,9 +168,13 @@ export async function createSubmission(submissionData: SubmissionParameters): Pr
     throw new InvalidInputError(`Invalid task id: ${taskTokenData.taskId}`);
   }
 
+  const mode = submissionData.userTests && submissionData.userTests.length ? SubmissionMode.UserTest : SubmissionMode.Submitted;
+
   let answerTokenData: PlatformAnswerTokenData|null = null;
   if (submissionData.answerToken) {
     answerTokenData = await extractPlatformAnswerTaskTokenData(submissionData.answerToken, submissionData.platform, submissionData.taskId);
+  } else if (!appConfig.testMode.enabled && 'UserTest' !== mode && '0' !== String(taskTokenData.payload.idUser)) {
+    throw new InvalidInputError('Missing answerToken, required for this type of submission');
   }
 
   const submissionParamsKeys: (keyof PlatformTaskTokenPayload)[] = [
@@ -188,8 +192,6 @@ export async function createSubmission(submissionData: SubmissionParameters): Pr
 
   checkAnswerTokenValid(answerTokenData, taskTokenData);
 
-  const mode = submissionData.userTests && submissionData.userTests.length ? SubmissionMode.UserTest : SubmissionMode.Submitted;
-
   // save source code (with bSubmission = 1)
   const idNewSourceCode = getRandomId();
   const idSubmission = getRandomId();