diff --git a/crates/calc/src/tests.rs b/crates/calc/src/tests.rs
index 4827b1bcc..357249f56 100644
--- a/crates/calc/src/tests.rs
+++ b/crates/calc/src/tests.rs
@@ -1193,6 +1193,24 @@ fn lookup_of_an_empty_cell_stays_blank() {
     }
 }
 
+#[test]
+fn blank_lookup_values_match_excel_windows_observation() {
+    // Excel 365 Windows 2609, build 16.0.20430.20032, black-box check 2026-10-13:
+    // A1="k", B1 and A2 genuinely empty.
+    // All three value lookups preserve blankness inside ISBLANK/concatenation.
+    let mut t = T::new();
+    t.set("A1", "k");
+    for (at, f, want) in [
+        ("D1", "=ISBLANK(VLOOKUP(\"k\",A1:B1,2,FALSE))", Value::Bool(true)),
+        ("D2", "=ISBLANK(HLOOKUP(\"k\",A1:A2,2,FALSE))", Value::Bool(true)),
+        ("D3", "=ISBLANK(LOOKUP(\"k\",A1:A1,B1:B1))", Value::Bool(true)),
+        ("D4", "=\"x\"&HLOOKUP(\"k\",A1:A2,2,FALSE)&\"y\"", Value::from("xy")),
+    ] {
+        t.set(at, f);
+        assert_eq!(t.get(at), want, "{f}");
+    }
+}
+
 #[test]
 fn cell_width_is_the_column_width_dialog_value_rounded() {
     // #735: Excel 365 values. Widths in px as `home.columnWidth {"chars"}` sets them