IB 1.5 in IB Express.

I have a table where I need the maximum value from. I ran "select
max(LastUpdated) from TableName" SQL against it and got the following
stats:

Plan
PLAN (TAXYEAR NATURAL)

Adapted Plan
PLAN (TAXYEAR NATURAL)

------ Performance info ------
Prepare time = 0ms
Execute time = 15ms
Avg fetch time = 15.00 ms
Current memory = 742,144
Max memory = 877,436
Memory buffers = 2,048
Reads from disk to cache = 0
Writes from cache to disk = 6
Fetches from cache = 392

Then I created an index on LastUpdated:

Create Index TableNameIdx1 on TableName(LastModified);

and when I rerand the select max()... against it, it is still using
the natural plan:

Plan
PLAN (TAXYEAR NATURAL)

Adapted Plan
PLAN (TAXYEAR NATURAL) // Why natural if an index exists?

------ Performance info ------
Prepare time = 0ms
Execute time = 0ms
Avg fetch time = 0.00 ms
Current memory = 812,408
Max memory = 947,700
Memory buffers = 2,048
Reads from disk to cache = 0
Writes from cache to disk = 6
Fetches from cache = 392

Both use the natural plan, bot have the same number of fetches from
the cache. Shouldn't it use the index in the second case? Am I
misunderstanding what is happening?

Thanks
Ed Dressel