HeaderToField$Key STRING is not a supported type

[arnitolog]

Hello,

I'm trying to copy the Header value to the key with the transformation below.

    transforms: "CreateDocID"
    transforms.CreateDocID.type: "com.github.jcustenborder.kafka.connect.transform.common.HeaderToField$Key"
    transforms.CreateDocID.header.mappings: "DocId:STRING"

according to the documentation It should work, but I get an error:
Caused by: java.lang.UnsupportedOperationException: STRING is not a supported type.

Is it possible to use this transformation to change the key with value from the header?