The template code for the `check_solution`

function gives you this code:

```
# Test if the row from the square matrix has only zeros (do not replcae the part 'is None')
if get_first_non_zero_value_from_row(None, None) is None:
# The matrix is singular, analyze the corresponding constant term to determine the type of solution
singular = True
```

The problem is that the real name of that function is `get_index_first_non_zero_value_from_row`

. I kept getting “function not defined” errors even though I was sure I had run the previous cells.