diff --git a/.github/workflows/polyval.yml b/.github/workflows/polyval.yml
index f65af6d..1ac5450 100644
--- a/.github/workflows/polyval.yml
+++ b/.github/workflows/polyval.yml
@@ -177,7 +177,9 @@ jobs:
           - x86_64-unknown-linux-gnu
     steps:
       - uses: actions/checkout@v6
-      - uses: dtolnay/rust-toolchain@nightly
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: nightly-2026-02-11 # pinned due to rust-lang/miri#4855
       - run: rustup component add miri && cargo miri setup
       - run: cargo miri test --target ${{ matrix.target }} --lib
 
diff --git a/polyval/src/backend/soft.rs b/polyval/src/backend/soft.rs
index 5e94c4f..26605fc 100644
--- a/polyval/src/backend/soft.rs
+++ b/polyval/src/backend/soft.rs
@@ -9,7 +9,8 @@ use crate::{Block, Key, ParBlocks, Tag, field_element::FieldElement};
 use zeroize::Zeroize;
 
 /// State of a POLYVAL hash operation.
-#[derive(Clone, Copy)]
+#[derive(Clone)]
+#[allow(missing_copy_implementations)]
 pub(crate) struct State {
     /// Hash key: fixed element of GF(2^128) that parameterizes the POLYVAL universal hash function.
     ///