diff --git a/contracts/credential-nft/src/mint.rs b/contracts/credential-nft/src/mint.rs
index 8a0be03..73b4780 100644
--- a/contracts/credential-nft/src/mint.rs
+++ b/contracts/credential-nft/src/mint.rs
@@ -64,6 +64,11 @@ pub fn mint_credential(
     score: u32,
     metadata_uri: &Symbol,
 ) -> u64 {
+    let zero_address = chainlearn_shared::zero_address(env);
+    if *to == zero_address {
+        panic!("cannot mint to zero address");
+    }
+
     // Metadata URI gate: must be non-empty, >= 8 chars, with a valid scheme
     validate_metadata_uri(env, metadata_uri);
 
diff --git a/contracts/learn-token/src/lib.rs b/contracts/learn-token/src/lib.rs
index ed1865a..9e6a591 100644
--- a/contracts/learn-token/src/lib.rs
+++ b/contracts/learn-token/src/lib.rs
@@ -1624,6 +1624,11 @@ impl LearnToken {
         let admin = storage::get_admin(&env);
         admin.require_auth();
 
+        let zero_address = chainlearn_shared::zero_address(&env);
+        if beneficiary == zero_address {
+            panic!("cannot vest to zero address");
+        }
+
         if total_amount <= 0 {
             panic!("total_amount must be positive");
         }
diff --git a/contracts/progress-tracker/src/lib.rs b/contracts/progress-tracker/src/lib.rs
index 8fb92da..742533c 100644
--- a/contracts/progress-tracker/src/lib.rs
+++ b/contracts/progress-tracker/src/lib.rs
@@ -2066,6 +2066,11 @@ impl ProgressTracker {
         Self::require_not_paused(&env);
         learner.require_auth();
 
+        let zero_address = chainlearn_shared::zero_address(&env);
+        if delegate == zero_address {
+            panic!("cannot delegate to zero address");
+        }
+
         types::write_entry(
             &env,
             &ProgressTrackerDataKey::DelegatedTo(learner.clone()),
@@ -2099,10 +2104,7 @@ impl ProgressTracker {
         Self::require_not_paused(&env);
         learner.require_auth();
 
-        types::remove_entry(
-            &env,
-            &ProgressTrackerDataKey::DelegatedTo(learner.clone()),
-        );
+        types::remove_entry(&env, &ProgressTrackerDataKey::DelegatedTo(learner.clone()));
 
         env.events()
             .publish((Symbol::new(&env, "delegation_revoked"),), (&learner,));
diff --git a/tests/unit/credential_tests.rs b/tests/unit/credential_tests.rs
index 46c5c3b..bdad438 100644
--- a/tests/unit/credential_tests.rs
+++ b/tests/unit/credential_tests.rs
@@ -564,7 +564,8 @@ mod credential_unit_tests {
         let learner = Address::generate(&env);
         let course_id = Symbol::new(&env, "rust_101");
         enrolled_and_completed_with_score(&env, &tracker_id, &learner, &course_id, 85);
-        let cred_id = client.mint_credential(&learner, &course_id, &85, &Symbol::new(&env, "ipfs_Qm123"));
+        let cred_id =
+            client.mint_credential(&learner, &course_id, &85, &Symbol::new(&env, "ipfs_Qm123"));
 
         let before = client.get_storage_size();
         let _ = client.verify_credential(&cred_id);
@@ -613,6 +614,36 @@ mod credential_unit_tests {
         assert_ne!(cert1, cert3);
 
         // Valid credential metadata URI format
-        assert!(client.try_mint_credential(&learner1, &course1, &90, &cert1).is_ok());
+        assert!(client
+            .try_mint_credential(&learner1, &course1, &90, &cert1)
+            .is_ok());
+    }
+
+    // ── Zero address rejection tests (#638) ──────────────────────────────
+
+    #[test]
+    #[should_panic(expected = "cannot mint to zero address")]
+    fn test_mint_credential_to_zero_address_panics() {
+        let env = Env::default();
+        let (_admin, contract_id, _) = setup_contract(&env);
+        let client = CredentialNftClient::new(&env, &contract_id);
+        env.mock_all_auths();
+
+        let zero_address = chainlearn_shared::zero_address(&env);
+        let course_id = Symbol::new(&env, "rust_101");
+        let uri = Symbol::new(&env, "ipfs_valid_metadata_123");
+        client.mint_credential(&zero_address, &course_id, &85, &uri);
+    }
+
+    #[test]
+    #[should_panic(expected = "cannot transfer admin to zero address")]
+    fn test_transfer_admin_to_zero_address_panics() {
+        let env = Env::default();
+        let (_admin, contract_id, _) = setup_contract(&env);
+        let client = CredentialNftClient::new(&env, &contract_id);
+        env.mock_all_auths();
+
+        let zero_address = chainlearn_shared::zero_address(&env);
+        client.transfer_admin(&zero_address);
     }
 }
diff --git a/tests/unit/progress_tests.rs b/tests/unit/progress_tests.rs
index a434a7f..74dd869 100644
--- a/tests/unit/progress_tests.rs
+++ b/tests/unit/progress_tests.rs
@@ -2414,11 +2414,15 @@ mod progress_unit_tests {
         client.emergency_pause();
 
         // enroll_checked must fail when paused
-        assert!(client.try_enroll_checked(&learner, &course_id, &None).is_err());
+        assert!(client
+            .try_enroll_checked(&learner, &course_id, &None)
+            .is_err());
 
         // Unpause and verify enroll_checked succeeds
         client.unpause();
-        assert!(client.try_enroll_checked(&learner, &course_id, &None).is_ok());
+        assert!(client
+            .try_enroll_checked(&learner, &course_id, &None)
+            .is_ok());
     }
 
     #[test]
@@ -2440,9 +2444,52 @@ mod progress_unit_tests {
         client.emergency_pause();
 
         let caller = learner.clone();
-        assert!(client.try_retake_quiz_for(&caller, &learner, &course_id, &Symbol::new(&env, "quiz_1"), &80).is_err());
+        assert!(client
+            .try_retake_quiz_for(
+                &caller,
+                &learner,
+                &course_id,
+                &Symbol::new(&env, "quiz_1"),
+                &80
+            )
+            .is_err());
 
         client.unpause();
-        assert!(client.try_retake_quiz_for(&caller, &learner, &course_id, &Symbol::new(&env, "quiz_1"), &80).is_ok());
+        assert!(client
+            .try_retake_quiz_for(
+                &caller,
+                &learner,
+                &course_id,
+                &Symbol::new(&env, "quiz_1"),
+                &80
+            )
+            .is_ok());
+    }
+
+    // ── Zero address rejection tests (#638) ──────────────────────────────
+
+    #[test]
+    #[should_panic(expected = "cannot transfer admin to zero address")]
+    fn test_transfer_admin_to_zero_address_panics() {
+        let env = Env::default();
+        let (_admin, contract_id) = setup_contract(&env);
+        let client = ProgressTrackerClient::new(&env, &contract_id);
+        env.mock_all_auths();
+
+        let zero_address = chainlearn_shared::zero_address(&env);
+        client.transfer_admin(&zero_address);
+    }
+
+    #[test]
+    #[should_panic(expected = "cannot delegate to zero address")]
+    fn test_delegate_progress_to_zero_address_panics() {
+        let env = Env::default();
+        let (_admin, contract_id) = setup_contract(&env);
+        let client = ProgressTrackerClient::new(&env, &contract_id);
+        env.mock_all_auths();
+
+        let learner = Address::generate(&env);
+        let zero_address = chainlearn_shared::zero_address(&env);
+        client.delegate_progress(&learner, &zero_address);
     }
 }
diff --git a/tests/unit/token_tests.rs b/tests/unit/token_tests.rs
index 67433ba..5161185 100644
--- a/tests/unit/token_tests.rs
+++ b/tests/unit/token_tests.rs
@@ -891,8 +891,6 @@ mod token_unit_tests {
     #[test]
     #[should_panic(expected = "no snapshot available at specified ledger")]
     fn test_vote_rejects_missing_snapshot_instead_of_using_current_balance() {
-    #[should_panic(expected = "contract is paused")]
-    fn test_create_proposal_fails_while_paused() {
         let env = Env::default();
         let (admin, contract_id, _) = setup_token(&env);
         let client = LearnTokenClient::new(&env, &contract_id);
@@ -928,6 +926,16 @@ mod token_unit_tests {
             &0,
             &1_000,
             &100,
+        );
+    }
+
+    #[test]
+    #[should_panic(expected = "contract is paused")]
+    fn test_create_proposal_fails_while_paused() {
+        let env = Env::default();
+        let (admin, contract_id, _) = setup_token(&env);
+        let client = LearnTokenClient::new(&env, &contract_id);
+        env.mock_all_auths();
         client.pause(&admin);
         client.create_proposal(
             &SorobanString::from_str(&env, "Paused proposal"),
@@ -939,7 +947,6 @@ mod token_unit_tests {
     }
 
     #[test]
-    fn test_vesting_schedule_cliff_linear_vesting_and_claiming() {
     #[should_panic(expected = "contract is paused")]
     fn test_vote_fails_while_paused() {
         let env = Env::default();
@@ -1841,4 +1848,30 @@ mod token_unit_tests {
         let beneficiary = Address::generate(&env);
         client.create_vesting(&beneficiary, &11, &0, &100);
     }
+
+    // ── Zero address rejection tests (#638) ──────────────────────────────
+
+    #[test]
+    #[should_panic(expected = "cannot transfer admin to zero address")]
+    fn test_transfer_admin_to_zero_address_panics() {
+        let env = Env::default();
+        let (_admin, contract_id, _) = setup_token(&env);
+        let client = LearnTokenClient::new(&env, &contract_id);
+        env.mock_all_auths();
+
+        let zero_address = chainlearn_shared::zero_address(&env);
+        client.transfer_admin(&zero_address);
+    }
+
+    #[test]
+    #[should_panic(expected = "cannot vest to zero address")]
+    fn test_create_vesting_to_zero_address_panics() {
+        let env = Env::default();
+        let (_admin, contract_id, _) = setup_token(&env);
+        let client = LearnTokenClient::new(&env, &contract_id);
+        env.mock_all_auths();
+
+        let zero_address = chainlearn_shared::zero_address(&env);
+        client.create_vesting(&zero_address, &1_000, &0, &100);
+    }
 }