I'm trying to extract data from `blosum62` matrix 

I have done the following code:

```
from Bio.SubsMat import MatrixInfo    
blosum = MatrixInfo.blosum62    
blosum['N','D'] 

```

et gets following error:

> 
> Traceback (most recent call last):  
> 
>  File "<stdin>", line 1, in <module>  
> 
>  KeyError: ('N', 'D')
> 
> 
> 

But in `blosum62` matrix there is a value of `('N','D') = 1`

Why is it giving KeyError ?