A method of enhanced error handling for 5G QoS operations associated with a PDU session is proposed. A QoS operation can be received via a PDU session modification procedure. In one example, the QoS operation is to create a new QoS rule for a PDU session, however, there is an existing QoS rule having the same QoS rule ID (QRI). The UE detects whether the QoS operation will causing the PDU session have no default QoS rule or have more than one default QoS rules. If the existing QoS rule is a default QoS rule, or if the new QoS rule have a default QoS rule (DQR) indication bit set to be true, then the UE rejects the QoS operation and indicates a QoS operation error to the network.