We could not complete your purchase

I have a lightly used Core 2 Duo MacBook that I thought I’d upgrade to Mountain Lion, but I encountered an unexpected issue in the App Store: “We could not complete your purchase. OS X Mountain Lion is not compatible with this computer.” Requirements: “OS X 10.6.8 or later” About This Mac says “Mac OS X Version 10.6.8” How did Tim know that I didn’t need this and should be doing something productive?