diff --git a/src/borg/fuse.py b/src/borg/fuse.py
index 34fa4008a..565d3b917 100644
--- a/src/borg/fuse.py
+++ b/src/borg/fuse.py
@@ -560,8 +560,10 @@ class FuseOperations(llfuse.Operations, FuseBackend):
         dir_gid = self.gid_forced if self.gid_forced is not None else self.default_gid
         dir_user = uid2user(dir_uid)
         dir_group = gid2group(dir_gid)
-        assert isinstance(dir_user, str)
-        assert isinstance(dir_group, str)
+        if not isinstance(dir_user, str):
+            raise Error(f"uid {dir_uid} can not be resolved to a username. Please check that the corresponding user exists or do not specify a uid mount option.")
+        if not isinstance(dir_group, str):
+            raise Error(f"gid {dir_gid} can not be resolved to a group name. Please check that the corresponding group exists or do not specify a gid mount option.")
         dir_mode = 0o40755 & ~self.umask
         self.default_dir = Item(mode=dir_mode, mtime=int(time.time() * 1e9),
                                 user=dir_user, group=dir_group, uid=dir_uid, gid=dir_gid)