The fallback for zero-scrobble data (e.g. a date-range filter with no matches) was an object literal ({year: "N/A", count: 0}), but the code that consumes it indexes with [0]/[1] as if it were the [key, count] tuple sortedYears/monthlyCounts entries actually are. On that fallback path it silently produced {year: undefined, count: undefined} instead of the intended placeholder.