diff --git a/src/addon/mod/assign/providers/prefetch-handler.ts b/src/addon/mod/assign/providers/prefetch-handler.ts
index d7a86836a..93ecb3f68 100644
--- a/src/addon/mod/assign/providers/prefetch-handler.ts
+++ b/src/addon/mod/assign/providers/prefetch-handler.ts
@@ -392,7 +392,7 @@ export class AddonModAssignPrefetchHandler extends CoreCourseActivityPrefetchHan
         // Prefetch feedback.
         if (submission.feedback) {
             // Get profile and image of the grader.
-            if (submission.feedback.grade && submission.feedback.grade.grader) {
+            if (submission.feedback.grade && submission.feedback.grade.grader && submission.feedback.grade.grader != -1) {
                 userIds.push(submission.feedback.grade.grader);
             }
 
diff --git a/src/core/user/providers/user.ts b/src/core/user/providers/user.ts
index 2a4d83d47..242c586d6 100644
--- a/src/core/user/providers/user.ts
+++ b/src/core/user/providers/user.ts
@@ -397,7 +397,7 @@ export class CoreUserProvider {
             userId = Number(userId); // Make sure it's a number.
 
             // Prevent repeats and errors.
-            if (!isNaN(userId) && !treated[userId]) {
+            if (!isNaN(userId) && !treated[userId] && userId > 0) {
                 treated[userId] = true;
 
                 promises.push(this.getProfile(userId, courseId, false, siteId).then((profile) => {