diff --git a/ssl/ssl_key_share.cc b/ssl/ssl_key_share.cc
index 497fedb99e..6912a2ebf3 100644
--- a/ssl/ssl_key_share.cc
+++ b/ssl/ssl_key_share.cc
@@ -87,7 +87,7 @@ class ECKeyShare : public SSLKeyShare {
         !EC_POINT_oct2point(group_, peer_point.get(), peer_key.data(),
                             peer_key.size(), /*ctx=*/nullptr)) {
       OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_ECPOINT);
-      *out_alert = SSL_AD_DECODE_ERROR;
+      *out_alert = SSL_AD_ILLEGAL_PARAMETER;
       return false;
     }
 
@@ -153,7 +153,7 @@ class X25519KeyShare : public SSLKeyShare {
 
     if (peer_key.size() != 32 ||
         !X25519(secret.data(), private_key_, peer_key.data())) {
-      *out_alert = SSL_AD_DECODE_ERROR;
+      *out_alert = SSL_AD_ILLEGAL_PARAMETER;
       OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_ECPOINT);
       return false;
     }
@@ -286,7 +286,7 @@ class KEMKeyShare : public SSLKeyShare {
       EVP_PKEY_kem_new_raw_public_key(nid_, peer_key.begin(), peer_key.size()));
 
     if (!pkey) {
-      *out_alert = SSL_AD_DECODE_ERROR;
+      *out_alert = SSL_AD_ILLEGAL_PARAMETER;
       OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
       return false;
     }
@@ -387,7 +387,7 @@ class KEMKeyShare : public SSLKeyShare {
                               &secret_bytes_written, ciphertext,
                               peer_key.size()) ||
                               secret_bytes_written != secret_len) {
-      OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_KEM_CIPHERTEXT);
+      OPENSSL_PUT_ERROR(SSL, SSL_AD_ILLEGAL_PARAMETER);
       return false;
     }
 
@@ -504,7 +504,7 @@ class HybridKeyShare : public SSLKeyShare {
         // Verify that |peer_key| contains enough data
         if (peer_key_read_index + component_key_size > peer_key.size()) {
           CBB_cleanup(&hybrid_shared_secret);
-          *out_alert = SSL_AD_DECODE_ERROR;
+          *out_alert = SSL_AD_ILLEGAL_PARAMETER;
           OPENSSL_PUT_ERROR(SSL, SSL_R_BAD_HYBRID_KEYSHARE);
           return false;
         }
@@ -612,7 +612,7 @@ class HybridKeyShare : public SSLKeyShare {
       // Final validation that |peer_key| was the correct size
       if (peer_key_index != peer_key.size()) {
         CBB_cleanup(&hybrid_shared_secret);
-        *out_alert = SSL_AD_DECODE_ERROR;
+        *out_alert = SSL_AD_ILLEGAL_PARAMETER;
         OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
         return false;
       }
diff --git a/ssl/ssl_test.cc b/ssl/ssl_test.cc
index 2c4b82483a..6a65f036eb 100644
--- a/ssl/ssl_test.cc
+++ b/ssl/ssl_test.cc
@@ -11905,7 +11905,7 @@ TEST_P(BadKemKeyShareAcceptTest, BadKemKeyShareAccept) {
     EXPECT_FALSE(server_key_share->Accept(&server_out_public_key,
                                           &server_secret, &server_alert,
                                           client_public_key));
-    EXPECT_EQ(server_alert, SSL_AD_DECODE_ERROR);
+    EXPECT_EQ(server_alert, SSL_AD_ILLEGAL_PARAMETER);
     CBB_cleanup(&server_out_public_key);
     CBB_cleanup(&client_out_public_key);
   }
@@ -11935,7 +11935,7 @@ TEST_P(BadKemKeyShareAcceptTest, BadKemKeyShareAccept) {
     EXPECT_FALSE(server_key_share->Accept(&server_out_public_key,
                                           &server_secret, &server_alert,
                                           client_public_key));
-    EXPECT_EQ(server_alert, SSL_AD_DECODE_ERROR);
+    EXPECT_EQ(server_alert, SSL_AD_ILLEGAL_PARAMETER);
     CBB_cleanup(&server_out_public_key);
     CBB_cleanup(&client_out_public_key);
   }
@@ -12394,7 +12394,7 @@ TEST_P(BadHybridKeyShareAcceptTest, BadHybridKeyShareAccept) {
     EXPECT_FALSE(server_key_share->Accept(&server_out_public_key,
                                           &server_secret, &server_alert,
                                           client_public_key));
-    EXPECT_EQ(server_alert, SSL_AD_DECODE_ERROR);
+    EXPECT_EQ(server_alert, SSL_AD_ILLEGAL_PARAMETER);
     CBB_cleanup(&server_out_public_key);
   }
 
@@ -12422,7 +12422,7 @@ TEST_P(BadHybridKeyShareAcceptTest, BadHybridKeyShareAccept) {
     EXPECT_FALSE(server_key_share->Accept(&server_out_public_key,
                                           &server_secret, &server_alert,
                                           client_public_key));
-    EXPECT_EQ(server_alert, SSL_AD_DECODE_ERROR);
+    EXPECT_EQ(server_alert, SSL_AD_ILLEGAL_PARAMETER);
     CBB_cleanup(&server_out_public_key);
     CBB_cleanup(&client_out_public_key);
   }
@@ -12547,7 +12547,7 @@ TEST_P(BadHybridKeyShareAcceptTest, BadHybridKeyShareAccept) {
         // The Accept() functionality for the NIST curves (e.g. P256) is
         // written so that it will return failure if the key share is invalid.
         EXPECT_EQ(hybrid_group->component_group_ids[i], SSL_GROUP_SECP256R1);
-        EXPECT_EQ(server_alert, SSL_AD_DECODE_ERROR);
+        EXPECT_EQ(server_alert, SSL_AD_ILLEGAL_PARAMETER);
       }
 
       client_public_key_index += t.offer_share_sizes[i];
@@ -12713,7 +12713,7 @@ TEST_P(BadHybridKeyShareFinishTest, BadHybridKeyShareFinish) {
                                       CBB_len(&server_out_public_key));
 
     EXPECT_FALSE(client_key_share->Finish(&client_secret, &client_alert, server_public_key));
-    EXPECT_EQ(client_alert, SSL_AD_DECODE_ERROR);
+    EXPECT_EQ(client_alert, SSL_AD_ILLEGAL_PARAMETER);
 
     CBB_cleanup(&server_out_public_key);
     CBB_cleanup(&client_out_public_key);
@@ -12809,7 +12809,7 @@ TEST_P(BadHybridKeyShareFinishTest, BadHybridKeyShareFinish) {
         // The Finish() functionality for the NIST curves (e.g. P256) is
         // written so that it will return failure if the key share is invalid.
         EXPECT_EQ(hybrid_group->component_group_ids[i], SSL_GROUP_SECP256R1);
-        EXPECT_EQ(client_alert, SSL_AD_DECODE_ERROR);
+        EXPECT_EQ(client_alert, SSL_AD_ILLEGAL_PARAMETER);
       }
 
       server_public_key_index += t.accept_share_sizes[i];