diff --git a/snap7/server/__init__.py b/snap7/server/__init__.py
index f9b43686..1a652cc7 100644
--- a/snap7/server/__init__.py
+++ b/snap7/server/__init__.py
@@ -2705,10 +2705,9 @@ def _build_cotp_cc(self) -> bytes:
         pdu_size_param = struct.pack(">BBB", self.COTP_PARAM_PDU_SIZE, 1, self.tpdu_size)
         pdu_length = 6 + len(pdu_size_param)
         base_pdu = struct.pack(
-            ">BBBHHB",
+            ">BBHHB",
             pdu_length,  # PDU length
             self.COTP_CC,  # PDU type
-            0x00,  # Reserved / CDT
             self.dst_ref,  # Destination reference (client's source ref)
             self.src_ref,  # Source reference (our ref)
             0x00,  # Class/option
diff --git a/tests/test_server.py b/tests/test_server.py
index b7af39a3..6928bfc8 100644
--- a/tests/test_server.py
+++ b/tests/test_server.py
@@ -316,6 +316,17 @@ def test_server_area_management(self) -> None:
 
 @pytest.mark.server
 class TestServerISOConnectionLimits:
+    def test_connection_confirm_has_valid_length_and_tpdu_size(self) -> None:
+        client_socket = MagicMock()
+        connection = ServerISOConnection(client_socket)
+        connection.dst_ref = 0x000F
+        connection.tpdu_size = 0x09
+
+        connection_confirm = connection._build_cotp_cc()
+
+        assert connection_confirm == bytes.fromhex("09d0000f000100c00109")
+        assert connection_confirm[0] == len(connection_confirm) - 1
+
     def test_partial_frame_timeout_closes_connection(self) -> None:
         client_socket = MagicMock()
         client_socket.recv.side_effect = [b"\x03", TimeoutError()]