diff --git a/mockfirestore/query.py b/mockfirestore/query.py
index 179ac46..d14d1ef 100644
--- a/mockfirestore/query.py
+++ b/mockfirestore/query.py
@@ -145,7 +145,7 @@ def _compare_func(self, op: str) -> Callable[[T, T], bool]:
         elif op == 'in':
             return lambda x, y: x in y
         elif op == 'array_contains':
-            return lambda x, y: y in x
+            return lambda x, y: x is not None and y in x
         elif op == 'array_contains_any':
             return lambda x, y: any([val in y for val in x])