diff --git a/src/rust/src/backend/aead.rs b/src/rust/src/backend/aead.rs
index 2a6641afa..94a9e949a 100644
--- a/src/rust/src/backend/aead.rs
+++ b/src/rust/src/backend/aead.rs
@@ -19,13 +19,36 @@ fn check_length(data: &[u8]) -> CryptographyResult<()> {
     Ok(())
 }
 
+enum Aad<'a> {
+    List(&'a pyo3::types::PyList),
+}
+
+fn process_aad(
+    ctx: &mut openssl::cipher_ctx::CipherCtx,
+    aad: Option<Aad<'_>>,
+) -> CryptographyResult<()> {
+    if let Some(Aad::List(ads)) = aad {
+        for ad in ads.iter() {
+            let ad = ad.extract::<CffiBuf<'_>>()?;
+            check_length(ad.as_bytes())?;
+            ctx.cipher_update(ad.as_bytes(), None)?;
+        }
+    }
+
+    Ok(())
+}
+
 fn encrypt_value<'p>(
     py: pyo3::Python<'p>,
     mut ctx: openssl::cipher_ctx::CipherCtx,
     plaintext: &[u8],
+    aad: Option<Aad<'_>>,
     tag_len: usize,
     tag_first: bool,
 ) -> CryptographyResult<&'p pyo3::types::PyBytes> {
+    check_length(plaintext)?;
+    process_aad(&mut ctx, aad)?;
+
     Ok(pyo3::types::PyBytes::new_with(
         py,
         plaintext.len() + tag_len,
@@ -58,7 +81,10 @@ fn decrypt_value<'p>(
     py: pyo3::Python<'p>,
     mut ctx: openssl::cipher_ctx::CipherCtx,
     ciphertext: &[u8],
+    aad: Option<Aad<'_>>,
 ) -> CryptographyResult<&'p pyo3::types::PyBytes> {
+    process_aad(&mut ctx, aad)?;
+
     Ok(pyo3::types::PyBytes::new_with(py, ciphertext.len(), |b| {
         // AES SIV can error here if the data is invalid on decrypt
         let n = ctx
@@ -150,26 +176,17 @@ impl AesSiv {
     ) -> CryptographyResult<&'p pyo3::types::PyBytes> {
         let key_buf = self.key.extract::<CffiBuf<'_>>(py)?;
         let data_bytes = data.as_bytes();
+        let aad = associated_data.map(Aad::List);
 
         if data_bytes.is_empty() {
             return Err(CryptographyError::from(
                 pyo3::exceptions::PyValueError::new_err("data must not be zero length"),
             ));
         };
-        check_length(data_bytes)?;
-
         let mut ctx = openssl::cipher_ctx::CipherCtx::new()?;
         ctx.encrypt_init(Some(&self.cipher), Some(key_buf.as_bytes()), None)?;
 
-        if let Some(ads) = associated_data {
-            for ad in ads.iter() {
-                let ad = ad.extract::<CffiBuf<'_>>()?;
-                check_length(ad.as_bytes())?;
-                ctx.cipher_update(ad.as_bytes(), None)?;
-            }
-        }
-
-        encrypt_value(py, ctx, data_bytes, 16, true)
+        encrypt_value(py, ctx, data_bytes, aad, 16, true)
     }
 
     fn decrypt<'p>(
@@ -180,12 +197,7 @@ impl AesSiv {
     ) -> CryptographyResult<&'p pyo3::types::PyBytes> {
         let key_buf = self.key.extract::<CffiBuf<'_>>(py)?;
         let data_bytes = data.as_bytes();
-
-        if data_bytes.is_empty() {
-            return Err(CryptographyError::from(
-                pyo3::exceptions::PyValueError::new_err("data must not be zero length"),
-            ));
-        }
+        let aad = associated_data.map(Aad::List);
 
         let mut ctx = openssl::cipher_ctx::CipherCtx::new()?;
         ctx.decrypt_init(Some(&self.cipher), Some(key_buf.as_bytes()), None)?;
@@ -199,16 +211,7 @@ impl AesSiv {
         let (tag, ciphertext) = data_bytes.split_at(16);
         ctx.set_tag(tag)?;
 
-        if let Some(ads) = associated_data {
-            for ad in ads.iter() {
-                let ad = ad.extract::<CffiBuf<'_>>()?;
-                check_length(ad.as_bytes())?;
-
-                ctx.cipher_update(ad.as_bytes(), None)?;
-            }
-        }
-
-        decrypt_value(py, ctx, ciphertext)
+        decrypt_value(py, ctx, ciphertext, aad)
     }
 }
 
diff --git a/tests/hazmat/primitives/test_aead.py b/tests/hazmat/primitives/test_aead.py
index 7db9607af..ce90f6892 100644
--- a/tests/hazmat/primitives/test_aead.py
+++ b/tests/hazmat/primitives/test_aead.py
@@ -681,7 +681,7 @@ class TestAESSIV:
         with pytest.raises(ValueError):
             aessiv.encrypt(b"", None)
 
-        with pytest.raises(ValueError):
+        with pytest.raises(InvalidTag):
             aessiv.decrypt(b"", None)
 
     def test_vectors(self, backend, subtests):