getFeeForMessage returns null value

I am following docs and making the following solana rpc call to devnet solana(https://api.devnet.solana.com) but I am getting null value in response

this is my body

{"id":1,"jsonrpc":"2.0","method":"getFeeForMessage","params":["AQABAgIAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEBAQAA"  ]}