diff --git a/crates/fidc-core/src/factor_events.rs b/crates/fidc-core/src/factor_events.rs
index 484f38f..f4c32ba 100644
--- a/crates/fidc-core/src/factor_events.rs
+++ b/crates/fidc-core/src/factor_events.rs
@@ -53,7 +53,7 @@ pub enum Expr {
     Operator {
         name: String,
         args: Vec<Expr>,
-        #[serde(default)]
+        #[serde(default, skip_serializing_if = "Option::is_none")]
         window: Option<usize>,
     },
 }
diff --git a/crates/fidc-core/src/pattern_context.rs b/crates/fidc-core/src/pattern_context.rs
index 6902045..9d6d7e1 100644
--- a/crates/fidc-core/src/pattern_context.rs
+++ b/crates/fidc-core/src/pattern_context.rs
@@ -248,6 +248,11 @@ mod tests {
     use super::*;
     use crate::{BenchmarkSnapshot, DailyFactorSnapshot, DailyMarketSnapshot, Instrument};
     use serde_json::json;
+    #[test]
+    fn normalized_rule_does_not_turn_an_omitted_window_into_explicit_null() {
+        let expression:Expr=serde_json::from_value(json!({"kind":"operator","name":"GT","args":[{"kind":"field","name":"close"},{"kind":"number","value":1}]})).unwrap();
+        assert!(serde_json::to_value(expression).unwrap().get("window").is_none());
+    }
     fn data(future: bool, reference: bool) -> DataSet {
         let mut days = vec![
             NaiveDate::from_ymd_opt(2026, 9, 4).unwrap(),