diff --git a/bumble/device.py b/bumble/device.py
index 7eb5dc15..32b06b8c 100644
--- a/bumble/device.py
+++ b/bumble/device.py
@@ -1578,15 +1578,12 @@ class CisParameters:
 
         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).
+            # SDU size is 0. If SDU size is 0, we set RTN to 0 as well for maintaining
+            # compatibility with older firmware that has error 0x30 bug.
             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]
diff --git a/tests/device_test.py b/tests/device_test.py
index 7df82802..fc21bf97 100644
--- a/tests/device_test.py
+++ b/tests/device_test.py
@@ -604,7 +604,7 @@ def test_cis_parameters_unidirectional():
     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)
+    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)
@@ -612,7 +612,7 @@ def test_cis_parameters_unidirectional():
     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)
+    assert cis_p2c.phy_c_to_p != hci.PhyBit(0)
 
 
 # -----------------------------------------------------------------------------