Master page and (child) page Page_Load event order

Hello!
My "Default.aspx" page is built using Site.Master as master page.
In Site.Master.cs I check for "null" value at Page_Load of one Session variable. If it's "null" I assign a value to it. Then I want to use it in Page_Load of the "Default.aspx". 
But I get an error for trying to use "null" value in "Default.aspx"...
Why?? Is the Page_Load of the "Default.aspx" loaded before the Page_Load of the master page??
Thx,
M
...