oh my, what am i doing

storage/mysql/mysql.go

@@ -261,14 +261,16 @@ func (s *Storage) ReadTile(ctx context.Context, level, index, minTreeSize uint64
 
 	// Do some validation to make sure we don't return data at a tree size we haven't reached yet
 	// Doing so could break caching at a higher level
-	entriesPerLevel := 1 << level * 8
+	startOfTileSize := (1 << ((1 + level) * 8)) * index // Size of perfect tree if this tile had 0 entries in it
+
 	numEntries := uint64(len(tile) / 32)
+	entriesPerPerfectSubtree := uint64(1 << (level * 8)) // Each leaf at this level implies this many permanent entries in the tree
 
-	minPermanentEntries := numEntries * uint64(entriesPerLevel)
+	minPermanentEntries := startOfTileSize + numEntries*uint64(entriesPerPerfectSubtree)
 
 	if minTreeSize > minPermanentEntries {
 		// If the user has requested a size larger than we have, they can't have it
-		return nil, os.ErrNotExist
+		return nil, fmt.Errorf("Requested tree size %d but tree size is %d: %w", minTreeSize, minPermanentEntries, os.ErrNotExist)
 	}
 
 	return tile, nil