‘too many values to unpack’, iterating over a dict. key=>string, value=>list

I am getting the too many values to unpack error. Any idea how I can fix this?

first_names = ['foo', 'bar']
last_names = ['gravy', 'snowman']

fields = {
    'first_names': first_names,
    'last_name': last_names,
}        

for field, possible_values in fields:  # error happens on this line