diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index bfbd9c9..51c1fad 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -16,6 +16,9 @@ on:
 
 name: CI
 
+env:
+  RUSTFLAGS: -Dwarnings
+
 jobs:
   msrv:
     runs-on: ubuntu-latest
diff --git a/authenticode/tests/test_errors.rs b/authenticode/tests/test_errors.rs
index e57754f..8f897c8 100644
--- a/authenticode/tests/test_errors.rs
+++ b/authenticode/tests/test_errors.rs
@@ -6,6 +6,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// Ignore warnings from not using the return value of `format!`.
+#![expect(unused_must_use)]
+
 use authenticode::{
     AttributeCertificateAuthenticodeError, AttributeCertificateError,
     AuthenticodeSignatureParseError, PeOffsetError,