Cannot Upgrade: "Version level of your database cannot be determined"

I want to upgrade my 1.3.9 to 1.5.0 but I'm not getting the upgrade option in the setup. I've followed the directions, but I get this error:

Previous Zen Cart® Installation Found
However, the version level of your database cannot be determined, usually resulting from wrong table prefixes, or other database settings mismatches.

CAUTION: Only use the Upgrade option if you are sure your configure.php settings are correct.

I've checked, double checked, and triple checked my configure.php files and they are correct. I've checked my table prefix and it is correct.

Any ideas or suggestions?

Thanks!

Josh Moree:

I've checked, double checked, and triple checked my configure.php files and they are correct. I've checked my table prefix and it is correct.!
There should be more than an "it". Your new and old database should match prefixes. If both are the same, then there is some other incorrect setting in the config files.

Have you done something like this first
http://www.zen-cart.com/content.php?148-how-do-i-upgrade-my-zen-cart-site
or are you "blindly" upgrading a live site and "hoping for the best"

The tables prefixes most likely don't match. I just had this issue today while upgrading several sites, for some sites the database tables had a prefix and some didn't have any prefix.
You need to first do a fresh installation, during the installation enter the prefix of the old database, if the old database doesn't have a prefix don't enter any.
After the installation use phpMyAdmin to import the old database into database you used for the new installation.
Then run the ZC installer again and it should now give the option to update the database.