SAP BPM:can not pass output data of a human activity to input of another activity

My Problem is

The output of the previous human activity is not passed to the input of next human activity.

Details

My demo is modified from the guide BPM OData: Implementing a Basic Custom Task Execution UI

I created a simple Process with two simple human activities.

the output mapping of human activity "Verify Customer Data" is below.

the input mapping of human activity "Add Credit Limit" is below.

after activity "Verify customer data" executed, I can see the output of the task

But the input of "Add Credit Limit" is empty.

NetWeaver Version

The version of NetWeaver is 7.3112