Words are inadequate

It wouldn't have been so bad, except that... hrm.
Okay. The way it works, you have four distinct symbols for 1-4 (empty space for the zero symbol). To represent the fives digit you take the symbol for 1-4, rotate it 90 degrees, and combine it with the symbol for the ones digit. The problem was that the symbol for one was a vertical line, so the symbol for five was a horizontal line, which didn't immediately trigger the "Oh, that's the symbol for 'one'" response. The order in which I found things did me in-- I realised "Oh, that's a horizontal line meaning 'add five'" and then assumed that the designers were lazy and including a base-ten system with only five digits plus the inclusion of the line.
Conclusion: Riven is smarter than I'd thought at first. This is good. :)