Which table first?

I have two tables; the first table has 2 million records, where 1 column is called DEA_NO, which 1 million records has DEA_NO = NULL.  The table has a non-clustered index on this column.  The second table has about 140 records, and a column called DEA_NO (same type and size as in table one).  Using the following query:

Update Table_Two
Set A.ME_Number = Table_One.ME_Number
From Table_Two A, Table_One(3)
Where A.DEA_NO = Table_One.DEA_NO

with ShowPlan ON, Sybase shows that it wants to use Table_One first (with the forced index) matching Table_Two with a table scan.  Unfortunately, this actually works like a table scan on Table_One anyway, only we are traversing the table in DEA_NO order.  Why does Sybase insist on using Table_One first, instead of moving through the 140 records in Table_Two, looking up the records (which are indexed) in Table_One.  

Thanks.

Brent