I'm stuck on is_int

My code gives the following error:

Oops, try again. Does your is_int function take exactly one argument (an integer)? Your code threw a "unsupported operand type(s) for -: 'int' and 'NoneType'" error.

I would expect it to calculate round down the number and then assign True or False.

```
def is_int(x):
def round_down(x):
z = round(x) - x
if z > 0:
y = x
else:
y = z - 1
if x - round_down(x) >= 0:
return False
else:
return True
```