crash when using string value for 'sources' in extension

[ItIsI-Orient]

When I have uploaded new pull request for detectron2, I have run into an error:

ERROR: test_pr_sha256_checksums (test.easyconfigs.easyconfigs.EasyConfigTest)
Make sure changed easyconfigs have SHA256 checksums in place.
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Traceback (most recent call last):
  File "test/easyconfigs/easyconfigs.py", line 909, in test_pr_sha256_checksums
    checksum_issues = check_sha256_checksums(retained_changed_ecs, whitelist=whitelist)
  File "/opt/hostedtoolcache/Python/2.7.18/x64/lib/python2.7/site-packages/easybuild/framework/easyconfig/tools.py", line 697, in check_sha256_checksums
    checksum_issues.extend(eb_class(ec).check_checksums())
  File "/opt/hostedtoolcache/Python/2.7.18/x64/lib/python2.7/site-packages/easybuild/easyblocks/generic/bundle.py", line 212, in check_checksums
    checksum_issues = super(Bundle, self).check_checksums()
  File "/opt/hostedtoolcache/Python/2.7.18/x64/lib/python2.7/site-packages/easybuild/framework/easyblock.py", line 2406, in check_checksums
    res = self.check_checksums_for(ext_opts, sub="of extension %s" % ext_name, source_cnt=1)
  File "/opt/hostedtoolcache/Python/2.7.18/x64/lib/python2.7/site-packages/easybuild/framework/easyblock.py", line 2355, in check_checksums_for
    for fn, checksum in zip(sources + es, checksums):
TypeError: cannot concatenate 'str' and 'list' objects

I have resolved this issue, but it didn't have anything to do with checksums (I forgot to put the sources value into brackets).