Solve the programming task below in a Python markdown code block.
*Debug*   function `getSumOfDigits` that takes positive integer to calculate sum of it's digits. Assume that argument is an integer.

### Example
```
123  => 6
223  => 7
1337 => 15
```
Also feel free to reuse/extend the following starter code:
```python
def get_sum_of_digits(num):
```