diff --git a/lit_nlp/app.py b/lit_nlp/app.py
index a83eabb7..76b6ddf4 100644
--- a/lit_nlp/app.py
+++ b/lit_nlp/app.py
@@ -14,6 +14,7 @@
 # ==============================================================================
 """LIT backend, as a standard WSGI app."""
 
+import collections
 from collections.abc import Callable, Iterable, Mapping, Sequence
 import functools
 import glob