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 CartŪ 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!