++ ln -s /tmpfs/DerivedData /Users/kbuilder/Library/Developer/Xcode/DerivedData
++ ln -sf /usr/local/bin/phpunit-5.7 /usr/local/bin/phpunit
+ tools/run_tests/run_tests_matrix.py -f basictests macos opt --exclude objc --internal_ci -j 1 --inner_jobs 4 --max_time=3600 --filter_pr_tests --base_branch origin/null
fatal: Not a valid object name origin/null
IMPORTANT: The changes you are testing need to be locally committed
because only the committed changes in the current branch will be
copied to the docker environment or into subworkspaces.
Looking for irrelevant tests to skip...
Finding file differences between gRPC origin/null branch and pull request...
Traceback (most recent call last):
  File "tools/run_tests/run_tests_matrix.py", line 564, in <module>
    relevant_jobs = filter_tests(jobs, args.base_branch)
  File "/Volumes/BuildData/tmpfs/src/github/grpc/tools/run_tests/python_utils/filter_pull_request_tests.py", line 184, in filter_tests
    changed_files = _get_changed_files(base_branch)
  File "/Volumes/BuildData/tmpfs/src/github/grpc/tools/run_tests/python_utils/filter_pull_request_tests.py", line 128, in _get_changed_files
    "HEAD"]).rstrip()
  File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/subprocess.py", line 223, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
subprocess.CalledProcessError: Command '['git', 'merge-base', 'origin/null', 'HEAD']' returned non-zero exit status 128
+ FAILED=true
+ ps aux