Why doesn't git recognize master branch?

[Chris Fouts]

I have the following simple Bitbucket (cloud) pipeline, where in this case the 

BITBUCKET_PR_DESTINATION_BRANCH = master

image: java:8
pipelines:
  pull-requests:
    'feature/*':
    - step:
        script:
        - git fetch "+refs/heads/*:refs/remotes/origin/*"
        - git diff $BITBUCKET_PR_DESTINATION_BRANCH ./gradle.properties > diff.txt

When it executes, I get

+ git diff $BITBUCKET_PR_DESTINATION_BRANCH ./gradle.properties > diff.txt
fatal: ambiguous argument 'master': unknown revision or path not in the working tree

Why so? 

[Chris Fouts · Accepted solution]

I tried this, and it worked.

git fetch origin $BITBUCKET_PR_DESTINATION_BRANCH:$BITBUCKET_PR_DESTINATION_BRANCH