What will the following program output?

mx = [[1, 3, 10], [], [2, 7, 5]]
first = []

for i in range(len(mx)):
  for j in range(len(mx[i])):
    if j == 0:
      first.append(mx[i][j])
print first

[1, 3, 10]
[1, 2]
Error message