I am trying to follow the material but I got an error :

```
def oracle_matrix(combo):
"""Return the oracle matrix for a secret combination.
Args:
combo (list[int]): A list of bits
representing a secret combination.
Returns:
array[float]: The matrix
representation of the oracle.
"""
index = np.array(np.ravel_multi_index(combo, [2]*len(combo)))
# Index of solution
my_array = np.identity(2**len(combo))
# Create the identity matrix
##################
# YOUR CODE HERE #
##################
# MODIFY DIAGONAL ENTRY CORRESPONDING TO SOLUTION INDEX
my_array[index-1][index-1] = -1 # modifying the index here by -1 gives error: Incorrect: your matrix doesn't look quite right.
return my_array
```

any idea