diff --git a/docs/performance.md b/docs/performance.md
index eb1861c8..0c2c99d2 100644
--- a/docs/performance.md
+++ b/docs/performance.md
@@ -2633,6 +2633,29 @@ else: 0))`; it now says `if first < 0 and n > 0, do: raise(ArgumentError,
   without `report: :full`).
 - **Status:** measured, provisional.
 
+### 6.42 A raising guard on the window drops every bounds check in rolling_stats' loop
+
+- **Change:** `mean_std` in `rolling_stats.exs` raises `ArgumentError` for a
+  window outside `1..length(x)`, one guard `if w < 1 or w > n` before the
+  loop. Past it `1 <= w <= n` (an `or` that is false narrows both sides),
+  which proves `x[i - w]` under `i >= w`, and `out[i - w + 1]` and
+  `out[m + i - w + 1]` with `m = n - w + 1 >= 1` and `length(out) == 2 * m`.
+- **The C** (fast-path report under `report: :full`): bounds checks kept
+  3 -> 0; the loop's flagged strip carried float, overflow and bounds checks
+  and now carries float checks only (the index arithmetic's overflow checks
+  went with the bounds facts). clang still does not vectorise the loop: its
+  remark lost "cannot identify array bounds" but keeps the sliding sums,
+  a recurrence ("value that could not be identified as reduction is used
+  outside the loop"), so no speed-up is expected from vector code. Not
+  timed: the stored row is stale until the maintainer asks for a new
+  measurement (docs/agents/runs.md).
+- **Behaviour:** a window of 0, a negative one or one longer than the input
+  raises `ArgumentError` at the call instead of failing later in the loop;
+  the example's check (one valid window) passes.
+- **Source:** `defk-o08.6`, [`rolling_stats.exs`](https://github.com/DanielLuning/defk/blob/main/examples/rolling_stats.exs); the
+  compiler side is `defk-9za` / `defk-b38` (6.34).
+- **Status:** C and report checked; not timed.
+
 ---
 
 ## 7. When a hand-written or wrapper NIF is still the answer
@@ -2940,4 +2963,4 @@ example or close a workload ticket, add its entries and its line here.
   `defk-sve`, `defk-sve.1` ([`state-copy-cost.md`](https://github.com/DanielLuning/defk/blob/main/docs/research/state-copy-cost.md)), `defk-ulk`, `defk-ulk.1` ([`simd.md`](https://github.com/DanielLuning/defk/blob/main/docs/research/simd.md)), `defk-33b`,
   `defk-ulk.4`, `defk-52v.15` ([`cdcl_sat.exs`](https://github.com/DanielLuning/defk/blob/main/examples/cdcl_sat.exs)), `defk-tif`, `defk-gqk`, `defk-6ag`,
   `defk-7dm` ([`bench/port_roundtrip.exs`](https://github.com/DanielLuning/defk/blob/main/bench/port_roundtrip.exs)), `defk-1ub`,
-  `defk-o08.2`, `defk-o08.3`, `defk-o08.4`, `defk-o08.5`.
+  `defk-o08.2`, `defk-o08.3`, `defk-o08.4`, `defk-o08.5`, `defk-o08.6`.
diff --git a/examples/rolling_stats.exs b/examples/rolling_stats.exs
index 880f6c6a..8fe4622c 100644
--- a/examples/rolling_stats.exs
+++ b/examples/rolling_stats.exs
@@ -10,7 +10,9 @@
 # step), so both versions start from the same bytes.
 #
 # Both sides return the same thing: one packed binary of the n - w + 1 means
-# followed by the n - w + 1 standard deviations (f64, native endian).
+# followed by the n - w + 1 standard deviations (f64, native endian). The
+# kernel raises ArgumentError for a window outside 1..n: that raising guard
+# is what lets the compiler drop every bounds check in its loop.
 #
 # Elixir baseline: optimised. Best of 7 runs after a warm-up, 1M samples,
 # w = 60, 12 schedulers, shared machine (provisional: other benchmarks were
@@ -52,9 +54,15 @@ defmodule Rolling.Kernels do
   use Defk
 
   # Sliding sums: O(n) regardless of window size. Returns means then stddevs,
-  # n - w + 1 values each, packed into one array.
+  # n - w + 1 values each, packed into one array. A window outside
+  # 1..length(x) raises ArgumentError.
   defk mean_std(x :: [f64], w :: i64) :: [f64] do
     n = length(x)
+
+    # a raising guard: past it 1 <= w <= n, which proves x[i - w] and both
+    # out writes in range (docs/language.md, "Preconditions: raising guards")
+    if w < 1 or w > n, do: raise(ArgumentError, "the window must be in 1..length(x)")
+
     m = n - w + 1
     out = alloc(f64, 2 * m)
 
diff --git a/test/non_finite_test.exs b/test/non_finite_test.exs
index 2a3c2554..d85af8cb 100644
--- a/test/non_finite_test.exs
+++ b/test/non_finite_test.exs
@@ -261,6 +261,38 @@ defmodule Defk.NonFiniteTest.K do
   end
 end
 
+# examples/rolling_stats.exs's mean_std before its raising guard on w: a bad
+# window reaches the loop, so a strip mixes a non-finite read with a read
+# past the end or a zero divisor
+defmodule Defk.NonFiniteTest.Rolling do
+  use Defk
+
+  defk mean_std(x :: [f64], w :: i64) :: [f64] do
+    n = length(x)
+    m = n - w + 1
+    out = alloc(f64, 2 * m)
+
+    _ =
+      for i <- 0..(n - 1)//1, reduce: {0.0, 0.0} do
+        {s, q} ->
+          v = x[i]
+          old = if i >= w, do: x[i - w], else: 0.0
+          s2 = s + v - old
+          q2 = q + v * v - old * old
+
+          if i >= w - 1 do
+            mean = s2 / w
+            out[i - w + 1] = mean
+            out[m + i - w + 1] = sqrt(max(q2 / w - mean * mean, 0.0))
+          end
+
+          {s2, q2}
+      end
+
+    out
+  end
+end
+
 defmodule Defk.NonFiniteTest do
   use ExUnit.Case, async: true
   use Defk.Properties
@@ -617,7 +649,9 @@ defmodule Defk.NonFiniteTest do
   end
 
   test "rolling mean_std raises its first error: a NaN read before the read past the end" do
-    rolling = fn x, w -> Defk.Test.assert_agrees(Rolling.Kernels, :mean_std, [x, w]) end
+    rolling = fn x, w ->
+      Defk.Test.assert_agrees(Defk.NonFiniteTest.Rolling, :mean_std, [x, w])
+    end
 
     for n <- [100, 2 * @strip + 3], p <- G.strip_positions(n), p < n - 1 do
       # w = -1 reads x[i + 1]: past the end at i = n - 1, after the NaN
@@ -631,11 +665,36 @@ defmodule Defk.NonFiniteTest do
     end
 
     # the window sums check the reads they add, once per strip (seen_later/1)
-    c = File.read!(String.replace_suffix(Rolling.Kernels.__defk_lib__(), ".so", ".c"))
+    c =
+      File.read!(String.replace_suffix(Defk.NonFiniteTest.Rolling.__defk_lib__(), ".so", ".c"))
+
     assert c =~ "defk_in_f64_later(ctx, "
     assert c =~ "#define defk_get_f64 DEFK_NF_get_f64"
   end
 
+  test "the catalog's mean_std raises on a bad window before reading, and keeps its strip checks" do
+    rolling = fn x, w -> Defk.Test.assert_agrees(Rolling.Kernels, :mean_std, [x, w]) end
+    n = 2 * @strip + 3
+
+    # its raising guard runs before the loop: no read, no zero divisor
+    for w <- [-1, 0, n + 1] do
+      assert {:raise, %ArgumentError{message: "the window must be in 1..length(x)"}} =
+               rolling.(with_bad(n, 5, 1.0e200), w)
+    end
+
+    for p <- G.strip_positions(n) do
+      assert {:raise, %ArgumentError{}} = rolling.(with_bad(n, p, :nan), 3)
+    end
+
+    assert {:ok, _} = rolling.(with_bad(n, -1, nil), n)
+
+    # past the guard every access is in range; the window sums still check
+    # the reads they add once per strip
+    c = File.read!(String.replace_suffix(Rolling.Kernels.__defk_lib__(), ".so", ".c"))
+    assert c =~ "defk_in_f64_later(ctx, "
+    refute c =~ "#define defk_get_f64 DEFK_NF_get_f64"
+  end
+
   test "mixed-error strips run flagged, with no unchecked access to a computed index" do
     c = File.read!(String.replace_suffix(K.__defk_lib__(), ".so", ".c"))
     assert c =~ "#define defk_get_in_f64 DEFK_NF_get_in_f64"