Empty string inserting a zero, not a null

My insert statement looks like this:

INSERT INTO foo (bar) VALUES ('');

The bar field was created like so:

bar INT(11)
    COLLATION: (NULL)
    NULL: YES
    DEFAULT: (NULL)

MySQL version: 5.1.

Shouldn’t an empty string insert a NULL? I’m not sure why I’m seeing a zero (0) being stored in the table.