Get phred quality for ab1 traces

Hello!
So I'm using python 3.5.5 and biopython 1.73, and I'm trying to get the phred quality scores for each peak in my .ab1 files. Unfortunately the following just returns an array of zeroes, which I know is false (given the scores I'm returned on the [Indigo website](https://gear.embl.de/indigo/)):
```
record = SeqIO.read('peak3_3.SP6.ab1', 'abi')
let_ann = record.letter_annotations['phred_quality']
print(let_ann)
```
What am I doing wrong?
Thanks!!