ValueError: could not convert string to float: 'Pencil'

[Balakay97]

def sortSales(filename):
    with open(filename, 'r') as f:
        f.readline() # skip header
        d = {}
        for line in f:
            if len(line) > 0:
                words = line.strip().split(',')
                name = words[1]
                sales = float(words[3]) * float(words[4])
                d[name] = sales
        items = [(sale, name) for name, sale in d.items()]
        items.sort()
        items.reverse()
        for sale, name in items:
            print('{:<10}: ${:>8,.2f}'.format(name, sale))

I'm getting an error saying ValueError: could not convert string to float: 'Pencil' for sales = float(words[3]) * float(words[4]. What do I need to do to fix it?