diff --git a/crates/engine/src/fill.rs b/crates/engine/src/fill.rs
index 9e0c970..9f24e00 100644
--- a/crates/engine/src/fill.rs
+++ b/crates/engine/src/fill.rs
@@ -350,7 +350,10 @@ pub fn series(cx: &mut Ctx, sheet: usize, r: RangeRef, rows: bool, kind: &str, s
                             let mut left = step.abs() as i64;
                             while left > 0 {
                                 n += step.signum();
-                                if datetime_from_serial(sys, n).is_some_and(|d| d.weekday != 0 && d.weekday != 6) {
+                                let Some(day) = datetime_from_serial(sys, n) else {
+                                    return Err(crate::EngineError::Other("The weekday series runs past the supported date range.".into()));
+                                };
+                                if day.weekday != 0 && day.weekday != 6 {
                                     left -= 1;
                                 }
                             }
diff --git a/crates/engine/src/tests.rs b/crates/engine/src/tests.rs
index 634f8e2..79617bc 100644
--- a/crates/engine/src/tests.rs
+++ b/crates/engine/src/tests.rs
@@ -2516,6 +2516,15 @@ fn fill_series_month_clamps_to_month_end() {
     assert_eq!(v(&s, "A4"), ymd(2026, 4, 15));
 }
 
+#[test]
+fn fill_series_weekday_out_of_range_step_errors() {
+    let mut s = s();
+    s.execute("cell.set", json!({"cell": "A1", "input": "45000"})).unwrap();
+    let r = s.execute("edit.fillSeries", json!({"range": "A1:A2", "direction": "columns", "type": "date", "dateUnit": "weekday", "step": 1e12}));
+    assert!(r.is_err());
+    assert_eq!(v(&s, "A2"), Value::Empty);
+}
+
 #[test]
 fn fill_series_month_leap_year() {
     let s = series_date("2028-01-31", "A1:A3", "month");