rest api gateway in front of dynamodb, POST mapping template for updateitem format

Hello,

I am trying to create a rest api with api gateway in front of a very simple dynamodb table with counters.

The table is just 2 fields: ucgkey01 as string, votecount as number

tested updates with

aws dynamodb update-item \
--table-name $tablename \
--key '{"ucgkey01": { "S": "testname01" }}' \
--update-expression "SET votecount = votecount + :incr" \
--expression-attribute-values '{":incr":{"N":"1"}}' \
--return-values UPDATED_NEW

I have a post method to dynamodb with UpdateItem action. I am trying this mapping template in the POST method of my api gw inspired by this

{
    "operation" : "UpdateItem",
	"TableName": "UGC-test01",
    "key" : {
        "ucgkey01" : { "S": "$input.path('$.ucgkey01')" }
    },
    "update" : {
        "expression" : "SET votecount = votecount + :incr",
        "expressionValues" : {
            ":incr":{"N":"1"}
        }
    }
}

but if I test a POST with

{

        "ucgkey01" : "testname01"
        
}

I get

{
  "__type": "com.amazon.coral.validate#ValidationException",
  "message": "1 validation error detected: Value null at 'key' failed to satisfy constraint: Member must not be null"
}

can someone please point me to a solution?

Thanks

a.