NOT NULL column behaves strangely

[B. Fongo]

Why does MySQL accept this insert statement with a warning?
 
Insert into Sessions (id) values (20de8376640263673ea03938);
 
Query OK, 1 row affected.
 
I expected a warning because of the Token column shouldn't be NULL! 
 
I have a table with three columns:
 
Id # int not null
Token # char (1) not null
SessTime # Timestamp