Postgresql, syntax error in IF clause

I'm trying to create a trigger that sets off a function that checks whether there is enough stock for a book when a shipment of the book is inserted into the table shipments. If there is not enough stock, an exception is raised, otherwise the stock is reduced by 1.

This is my code:

CREATE FUNCTION decstock() RETURNS trigger AS $pname$
    BEGIN

        IF (SELECT stock FROM stock WHERE stock.isbn = NEW.ISBN) = 0 THEN
            RAISE EXCEPTION 'There is no stock to ship'
        ELSE
            UPDATE stock
            SET stock = stock-1
            WHERE stock.isbn = NEW.isbn;
        END IF;
    END;
$pname$ LANGUAGE plpgsql;

CREATE TRIGGER hej
BEFORE UPDATE ON shipments
FOR EACH ROW /*Why on each row? Aren't we supposed to only look at the stock for the specified isbn? Also, how do I reference the stock table here?*/
EXECUTE PROCEDURE decstock();

I get an error when running this, it says:

ERROR:  syntax error at or near "ELSE"
LINE 7:   ELSE
          ^

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

ERROR: syntax error at or near "ELSE"
SQL state: 42601
Character: 181

I can't find the syntax error I have made here. Any tips?