Negative weight for boolean field if true


I'm currently using function scoring to weight our results. I'd like to perform a simple weighed downgrade on the score, if a boolean field is set to true.

I was hoping true may be interpreted at 1, but this gives an error:

                                ["field_value_factor" => [
                                  "field"=> "booleanfield",

                                  "modifier"=> "none",
                                    "factor" => 1


Is there any way to take this into consideration on the scoring (we dont wish to filter them out completely)