The function below keeps returning this error message. I thought that maybe the double_precision field type was what was causing this, and I tried to use CAST, but either that's not it, or I didn't do it right... Help?

Here's the error:

ERROR:  input is out of range
CONTEXT:  PL/pgSQL function "calculate_distance" line 7 at RETURN

********** Error **********

ERROR: input is out of range
SQL state: 22003
Context: PL/pgSQL function "calculate_distance" line 7 at RETURN

And here's the function:

 CREATE OR REPLACE FUNCTION calculate_distance(character varying, 
double precision, double precision, 
double precision, double precision)

      RETURNS double precision AS
    $BODY$ 
            DECLARE earth_radius double precision; 

            BEGIN 
                    earth_radius := 3959.0; 

                    RETURN earth_radius * acos(sin($2 / 57.2958) * 
sin($4 / 57.2958) + cos($2/ 57.2958) * cos($4 / 57.2958) 
* cos(($5 / 57.2958) - ($3 / 57.2958)));
            END; 
            $BODY$
      LANGUAGE 'plpgsql' VOLATILE
      COST 100;
    ALTER FUNCTION calculate_distance(character varying, 
double precision, double precision, double precision, 
double precision) OWNER TO postgres;



    //I tried changing (unsuccessfully) that RETURN line to: 

    RETURN CAST( (earth_radius * acos(sin($2 / 57.2958) 
* sin($4 / 57.2958) + cos($2/ 57.2958) * cos($4 / 57.2958) 
* cos(($5 / 57.2958) - ($3 / 57.2958))) ) AS text);