```
def sum10(a, b):
    if sum([a, b]) % 10 == 0: return True; return False

print sum10(7, 3)
print sum10(-13, -17)
print sum10(3, 8)

```

the result is:

```
True
True
None

```

not what I expected:

```
True
True
False

```

any idea?