How do I modify a MySQL column to allow NULL?

MySQL 5.0.45 What is the syntax to alter a table to allow a column to be null, alternately what's wrong with this: ALTER mytable MODIFY mycolumn varchar(255) null; I interpreted the manual as just run the above and it would recreate the column, this time allowing null. The server is telling me I have syntactical errors. I just don't see them.