diff --git a/server/szurubooru/func/posts.py b/server/szurubooru/func/posts.py
index 3863e5b..ee43b92 100644
--- a/server/szurubooru/func/posts.py
+++ b/server/szurubooru/func/posts.py
@@ -355,8 +355,13 @@ def update_post_tags(post, tag_names):
 
 def update_post_relations(post, new_post_ids):
     assert post
+    try:
+        new_post_ids = [int(id) for id in new_post_ids]
+    except ValueError:
+        raise InvalidPostRelationError(
+            'A relation must be numeric post ID.')
     old_posts = post.relations
-    old_post_ids = [p.post_id for p in old_posts]
+    old_post_ids = [int(p.post_id) for p in old_posts]
     new_posts = db.session \
         .query(db.Post) \
         .filter(db.Post.post_id.in_(new_post_ids)) \
diff --git a/server/szurubooru/tests/func/test_posts.py b/server/szurubooru/tests/func/test_posts.py
index 56daaee..9c5ecd0 100644
--- a/server/szurubooru/tests/func/test_posts.py
+++ b/server/szurubooru/tests/func/test_posts.py
@@ -521,6 +521,7 @@ def test_update_post_relations_bidirectionality(post_factory):
     db.session.flush()
     post = post_factory()
     posts.update_post_relations(post, [relation1.post_id, relation2.post_id])
+    db.session.flush()
     posts.update_post_relations(relation1, [])
     assert len(post.relations) == 1
     assert post.relations[0].post_id == relation2.post_id