Function's final statement must be a SELECT

I am trying to write a function in PgAdmin 1.8.2 for postgres 8.3. The function should check the input parameter for NULL. If its NULL then function should return 'Auto' otherwise return the input as such.

CREATE FUNCTION check_for_null( inp character varying) RETURNS character varying AS $$
    DECLARE
        temp1 character varying := 'Auto';
        temp2 character varying := inp;
    BEGIN
        
        IF temp2 IS NOT NULL THEN
        temp1 := temp2;
        END IF;   
       
        RETURN temp1;
    END;
$$ LANGUAGE SQL;

When I hit 'OK' in create function window, I get following error :- 

ERROR: return type mismatch in function declared to return character varying
DETAIL: Function's final statement must be a SELECT.
CONTEXT: SQL function "check_for_null"
Any suggestions why this happens ??

Thanks