diff --git a/src/packet/public_key.js b/src/packet/public_key.js
index a7de9be9d..e444f7504 100644
--- a/src/packet/public_key.js
+++ b/src/packet/public_key.js
@@ -138,6 +138,10 @@ class PublicKeyPacket {
       ) {
         throw new Error('Legacy curve25519 cannot be used with v6 keys');
       }
+      // The composite ML-KEM + ECDH schemes MUST be used only with v6 keys.
+      if (this.version !== 6 && this.algorithm === enums.publicKey.pqc_mlkem_x25519) {
+        throw new Error('Unexpected key version: ML-KEM algorithms can only be used with v6 keys');
+      }
       this.publicParams = publicParams;
       pos += read;
 
diff --git a/src/packet/secret_key.js b/src/packet/secret_key.js
index c10836b74..7356ac1e5 100644
--- a/src/packet/secret_key.js
+++ b/src/packet/secret_key.js
@@ -532,6 +532,9 @@ class SecretKeyPacket extends PublicKeyPacket {
     )) {
       throw new Error(`Cannot generate v6 keys of type 'ecc' with curve ${curve}. Generate a key of type 'curve25519' instead`);
     }
+    if (this.version !== 6 && this.algorithm === enums.publicKey.pqc_mlkem_x25519) {
+      throw new Error(`Cannot generate v${this.version} keys of type 'pqc'. Generate a v6 key instead`);
+    }
     const { privateParams, publicParams } = await crypto.generateParams(this.algorithm, bits, curve, symmetric);
     this.privateParams = privateParams;
     this.publicParams = publicParams;
diff --git a/test/crypto/validate.js b/test/crypto/validate.js
index fde15d3ea..97f7d3a94 100644
--- a/test/crypto/validate.js
+++ b/test/crypto/validate.js
@@ -81,7 +81,7 @@ async function cloneKeyPacket(key) {
 }
 
 async function generatePrivateKeyObject(options) {
-  const config = { rejectCurves: new Set() };
+  const config = { rejectCurves: new Set(), ...options.config };
   const { privateKey } = await openpgp.generateKey({ ...options, userIDs: [{ name: 'Test', email: 'test@test.com' }], format: 'object', config });
   return privateKey;
 }
@@ -317,7 +317,7 @@ export default () => {
   describe('PQC parameter validation', function() {
     let pqcEncryptionSubkey;
     before(async () => {
-      const key = await generatePrivateKeyObject({ type: 'symmetric', subkeys: [{ type: 'pqc' }] });
+      const key = await generatePrivateKeyObject({ type: 'symmetric', subkeys: [{ type: 'pqc', config: { v6Keys: true } }] });
       pqcEncryptionSubkey = key.subkeys[0];
     });
 
diff --git a/test/general/key.js b/test/general/key.js
index 65dcf6267..74fb27c06 100644
--- a/test/general/key.js
+++ b/test/general/key.js
@@ -4525,6 +4525,17 @@ I8kWVkXU6vFOi+HWvv/ira7ofJu16NnoUkhclkUrk0mXubZvyl4GBg==
       expect(v6Key.subkeys).to.have.length(1);
     });
 
+    it('should throw when trying to add a ML-KEM PQC key to a v4 key', async function() {
+      const v4Key = await openpgp.decryptKey({
+        privateKey: await openpgp.readKey({ armoredKey: priv_key_rsa }),
+        passphrase: 'hello world'
+      });
+      expect(v4Key.keyPacket.version).to.equal(4);
+      expect(v4Key.subkeys).to.have.length(1);
+      await expect(v4Key.addSubkey({ type: 'pqc', sign: false })).to.be.rejectedWith(/Cannot generate v4 keys of type 'pqc'/);
+      expect(v4Key.subkeys).to.have.length(1);
+    });
+
     it('should throw when trying to encrypt a subkey separately from key', async function() {
       const privateKey = await openpgp.decryptKey({
         privateKey: await openpgp.readKey({ armoredKey: priv_key_rsa }),