求助 ADS differential port设置的问题

我在做耦合电感layout的时候，需要在电感两端加上差模和共模的激励，我使用了port editor中的differential port，但是，在设置associate with port number的时候，出现如下提示：
Port P2 cannot be associated with port number 1.
If they are in the same reference plane, and you want to associate them:
Please set both ports to the correct Port Type before
setting ^Associate with port number^
请问可能是什么原因，设置步骤我都按照manual中的步骤，应该没有问题。
manual中提到的differential port 要求
Differential ports should be used in situations where an electric field is likely to build up between two ports (odd modes propagate). This can occur when:

The two ports are close together There is no ground plane in the circuit or the ground plane is relatively far away One port behaves (to a degree) like a ground to the other port, and polarity between the ports is developed. The ports are connected to objects that are on strip metallization layers. 设置方法：

To define a differential port:

Choose Momentum > Port Editor . Select the port that you want to assign this type to. Note the port number. In the Port Editor dialog box, under Port Type, select Differential . Under Polarity, make sure that Normal is selected. Click Apply . Select the second port. In the Port Editor dialog box, under Port Type, select Differential . Under Polarity, select Reversed . Under Associate with port number, enter the number of the previously-selected port. Click Apply . Repeat these steps for other differential port pairs in the circuit. Click OK to dismiss the dialog box. 可能哪里出了问题，请大家赐教！谢谢！


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