diff --git a/ndc_stdlib/src/sequence.rs b/ndc_stdlib/src/sequence.rs
index 7a1069eb..ea6c7f8d 100644
--- a/ndc_stdlib/src/sequence.rs
+++ b/ndc_stdlib/src/sequence.rs
@@ -707,8 +707,10 @@ mod inner {
 
     /// Returns a list of all contiguous windows of `length` size. The windows overlap. If the `seq` is shorter than size, the iterator returns no values.
     #[function(return_type = Vec<_>)]
-    pub fn windows(seq: SeqValue, length: i64) -> anyhow::Result<Value> {
-        let length = length as usize;
+    pub fn windows(seq: SeqValue, length: usize) -> anyhow::Result<Value> {
+        if length == 0 {
+            return Err(anyhow!("window size must be non-zero"));
+        }
         Ok(Value::list(
             seq.try_into_iter()
                 .ok_or_else(|| anyhow!("windows requires a sequence"))?
diff --git a/tests/functional/programs/900_bugs/bug0024_windows_zero_size.ndc b/tests/functional/programs/900_bugs/bug0024_windows_zero_size.ndc
new file mode 100644
index 00000000..372e5172
--- /dev/null
+++ b/tests/functional/programs/900_bugs/bug0024_windows_zero_size.ndc
@@ -0,0 +1,5 @@
+// `windows(seq, 0)` used to reach `slice::windows(0)`, which panics with
+// "window size must be non-zero". It now returns a graceful error, matching
+// the sibling `chunks` function.
+// expect-error: window size must be non-zero
+windows([1, 2, 3], 0);