Most problems are detected when Inform translates the source text, but this is one of the exceptions, a "run-time problem" which emerges only during play.

This is a straightforward problem: you tried to look up a row which didn't exist. For instance, if there are three rows, then an attempt to look up an entry on row 4 will produce this problem. (What may be a little less straightforward is that the problem doesn't show itself when you choose a row, only when you make use of it.)