Why showing This Error - spiral forward i 10 IndentationError expected an indented block

Question

import turtle
sc=turtle.Screen()
sc.setup(600,600)
spiral=turtle.Turtle()
spiral.speed(10)

sc.bgcolor("black")

col=("yellow","blue","white","green")
c=0

for i in range(50):
spiral.forward(i*10)
spiral.right(144)
spiral.color(col[c])
If c==3:
else:
c+=1