diff --git a/backend/oasst_backend/exceptions.py b/backend/oasst_backend/exceptions.py
index 98224790..c1a7bd2e 100644
--- a/backend/oasst_backend/exceptions.py
+++ b/backend/oasst_backend/exceptions.py
@@ -34,6 +34,7 @@ class OasstErrorCode(IntEnum):
     INVALID_TASK_TYPE = 2004
     USER_NOT_SPECIFIED = 2005
     NO_THREADS_FOUND = 2006
+    NO_REPLIES_FOUND = 2007
     WORK_PACKAGE_NOT_FOUND = 2100
     WORK_PACKAGE_EXPIRED = 2101
     WORK_PACKAGE_PAYLOAD_TYPE_MISMATCH = 2102
diff --git a/backend/oasst_backend/prompt_repository.py b/backend/oasst_backend/prompt_repository.py
index 6d44c443..95e7867e 100644
--- a/backend/oasst_backend/prompt_repository.py
+++ b/backend/oasst_backend/prompt_repository.py
@@ -445,6 +445,8 @@ class PromptRepository:
 
         parent = parent.order_by(func.random()).limit(1).subquery()
         replies = self.db.query(Post).filter(Post.parent_id.in_(parent)).order_by(func.random()).limit(max_size).all()
+        if not replies:
+            raise OasstError("No replies found", OasstErrorCode.NO_REPLIES_FOUND)
 
         thread = self.fetch_thread(replies[0].thread_id)
         thread = {p.id: p for p in thread}