How to write a PL/pgSQL function to execute a dynamic query?

I am writing a function that needs to execute an UPDATE on every single row for a query. But it's not working. It doesn't loop over the rows at all. Can you help me spot my error? Here is what my function looks like:

CREATE OR REPLACE FUNCTION cs_parcel_postproc(cid int, props text) RETURNS integer AS $$
  DECLARE
    cid int;
    props text;
BEGIN
  RAISE NOTICE 'Postprocessing parcels...';

  EXECUTE 'UPDATE parcels SET ' || props || ' WHERE city_id =' || cid || ';';
  RAISE NOTICE 'Done post processing parcels.';
  RETURN 1;
END;
$$ LANGUAGE plpgsql;

EDIT: I discovered that cid is NULL inside the function. I don't understand how that could be.