diff --git a/bumble/device.py b/bumble/device.py
index e90dbf01..f3b431c5 100644
--- a/bumble/device.py
+++ b/bumble/device.py
@@ -1575,6 +1575,18 @@ class CisParameters:
         rtn_c_to_p: int = DEVICE_DEFAULT_ISO_CIS_RTN  # Number of C->P retransmissions
         rtn_p_to_c: int = DEVICE_DEFAULT_ISO_CIS_RTN  # Number of P->C retransmissions
 
+        def __post_init__(self) -> None:
+            # For unidirectional CIS (e.g., Central-to-Peripheral only), the unused direction's
+            # SDU size is 0. If SDU size is 0, the corresponding retransmission count and PHY
+            # must also be set to 0. Otherwise, some controllers will reject the parameters
+            # with error 0x30 (Parameter Out Of Mandatory Range).
+            if self.max_sdu_c_to_p == 0:
+                self.rtn_c_to_p = 0
+                self.phy_c_to_p = hci.PhyBit(0)
+            if self.max_sdu_p_to_c == 0:
+                self.rtn_p_to_c = 0
+                self.phy_p_to_c = hci.PhyBit(0)
+
     cig_id: int
     cis_parameters: list[CisParameters]
     sdu_interval_c_to_p: int  # C->P SDU interval, in microseconds
diff --git a/tests/device_test.py b/tests/device_test.py
index a45c43dd..3efe4db1 100644
--- a/tests/device_test.py
+++ b/tests/device_test.py
@@ -595,6 +595,25 @@ def on_hci_le_cis_established_event(host, event):
         await asyncio.wait_for(cis_create_task, _TIMEOUT)
 
 
+# -----------------------------------------------------------------------------
+def test_cis_parameters_unidirectional():
+    # Test C2P unidirectional (P to C not used)
+    cis_c2p = CigParameters.CisParameters(cis_id=1, max_sdu_p_to_c=0)
+    assert cis_c2p.max_sdu_c_to_p != 0
+    assert cis_c2p.rtn_c_to_p != 0
+    assert cis_c2p.phy_c_to_p != hci.PhyBit(0)
+    assert cis_c2p.rtn_p_to_c == 0
+    assert cis_c2p.phy_p_to_c == hci.PhyBit(0)
+
+    # Test P2C unidirectional (C to P not used)
+    cis_p2c = CigParameters.CisParameters(cis_id=2, max_sdu_c_to_p=0)
+    assert cis_p2c.max_sdu_p_to_c != 0
+    assert cis_p2c.rtn_p_to_c != 0
+    assert cis_p2c.phy_p_to_c != hci.PhyBit(0)
+    assert cis_p2c.rtn_c_to_p == 0
+    assert cis_p2c.phy_c_to_p == hci.PhyBit(0)
+
+
 # -----------------------------------------------------------------------------
 @pytest.mark.asyncio
 async def test_enter_and_exit_sniff_mode():