EventBridge pipe transformer not working for arrays

I'm trying to set up a simple transformer for an EventBridge pipe that connects a SQS queue to a Step Functions workflow. The SQS queue receives input in the form

[  
  {  
    "key1": "value1",  
    "key2": "value2"  
  }  
]

The workflow expects a similar format but different key identifiers:

[  
  {  
    "workflowKey1": "value1",  
    "workflowKey2": "value2"  
  }  
]

The workflow can also be invoked from other sources so changing the expected input is not easily possible. Clicking on the fields I want to extract in AWS console suggests the following transformer, which unfortunately only extracts empty strings:

{
  "workflowKey1": <$.0.key1>,
  "workflowKey2": <$.0.key2>
}

Is there something wrong with my transformer, or is this an issue with EventBridge pipe itself?