diff --git a/genlayer_py/contracts/actions.py b/genlayer_py/contracts/actions.py
index b40f9c6..b448419 100644
--- a/genlayer_py/contracts/actions.py
+++ b/genlayer_py/contracts/actions.py
@@ -884,6 +884,13 @@ def _encode_add_transaction_data(
     use_fee_aware_transaction = (
         transaction_fees["requires_fee_aware_transaction"] or abi_version == "fees"
     )
+    argument_count = len(contract_fn.argument_types)
+    if abi_version != "fees" and argument_count not in (5, 6):
+        raise ValueError(
+            "Unsupported addTransaction ABI: expected 5 or 6 arguments, "
+            f"got {argument_count}"
+        )
+
     normalized_valid_until = to_uint(
         valid_until,
         "valid_until",
@@ -909,8 +916,13 @@ def _encode_add_transaction_data(
         consensus_max_rotations,
         self.w3.to_bytes(hexstr=data),
     ]
-    if len(contract_fn.argument_types) >= 6:
+    if argument_count == 6:
         add_transaction_args.append(normalized_valid_until)
+    elif argument_count != 5:
+        raise ValueError(
+            "Unsupported addTransaction ABI: expected 5 or 6 arguments, "
+            f"got {argument_count}"
+        )
 
     params = abi_encode(
         contract_fn.argument_types,
diff --git a/tests/unit/contracts/test_contract_actions.py b/tests/unit/contracts/test_contract_actions.py
index 5a9e298..4d8bec6 100644
--- a/tests/unit/contracts/test_contract_actions.py
+++ b/tests/unit/contracts/test_contract_actions.py
@@ -70,6 +70,23 @@
     }
 ]
 
+ADD_TRANSACTION_ABI_V7 = [
+    {
+        "type": "function",
+        "name": "addTransaction",
+        "stateMutability": "nonpayable",
+        "inputs": [
+            {"name": "_sender", "type": "address"},
+            {"name": "_recipient", "type": "address"},
+            {"name": "_numOfInitialValidators", "type": "uint256"},
+            {"name": "_maxRotations", "type": "uint256"},
+            {"name": "_calldata", "type": "bytes"},
+            {"name": "_validUntil", "type": "uint256"},
+            {"name": "_futureField", "type": "uint256"},
+        ],
+        "outputs": [],
+    }
+]
 ADD_TRANSACTION_ABI_WITH_FEES = [
     {
         "type": "function",
@@ -632,6 +649,33 @@ def test_encode_add_transaction_uses_v6_signature_when_abi_has_6_inputs():
     assert encoded.startswith(f"0x{selector}")
 
 
+def test_encode_add_transaction_rejects_unknown_argument_count():
+    client = _make_client(ADD_TRANSACTION_ABI_V7)
+
+    with pytest.raises(ValueError, match="expected 5 or 6 arguments, got 7"):
+        contract_actions._encode_add_transaction_data(
+            self=client,
+            sender_account=client.local_account,
+            recipient=RECIPIENT,
+            consensus_max_rotations=3,
+            data="0x",
+        )
+
+
+def test_encode_add_transaction_rejects_v7_abi_when_fee_aware_transaction_is_requested():
+    client = _make_client(ADD_TRANSACTION_ABI_V7)
+
+    with pytest.raises(ValueError, match="expected 5 or 6 arguments, got 7"):
+        contract_actions._encode_add_transaction_data(
+            self=client,
+            sender_account=client.local_account,
+            recipient=RECIPIENT,
+            consensus_max_rotations=3,
+            data="0x",
+            transaction_fees={"requires_fee_aware_transaction": True},
+        )
+
+
 def test_encode_add_transaction_uses_fee_signature_when_abi_has_tuple_input():
     client = _make_client(ADD_TRANSACTION_ABI_WITH_FEES)