diff --git a/outlines/fsm/json_schema.py b/outlines/fsm/json_schema.py
index 04d16d672..f203fa132 100644
--- a/outlines/fsm/json_schema.py
+++ b/outlines/fsm/json_schema.py
@@ -136,7 +136,7 @@ def optimize_schema(instance):
             elif "anyOf" in subinstance and _has_null_type(subinstance["anyOf"]):
                 any_of_list = subinstance.pop("anyOf")
                 filtered_any_of_list = list(
-                    filter(lambda d: _is_null_type(d), any_of_list)
+                    filter(lambda d: not _is_null_type(d), any_of_list)
                 )
                 if len(filtered_any_of_list) == 0:
                     keys_to_remove.add(key)