debug USGAN

[AugustJW]

What does this PR do?

Fixing #338 (issue)

Thanks for the help from @susan1314！

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 8542481975

Details

• 17 of 17 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 80.826%

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Totals
Change from base Build 8526835352:	0.007%
Covered Lines:	6791
Relevant Lines:	8402

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💛 - Coveralls

[WenjieDu]

I think the code here is good for merging. I'll fix the linting issue on the dev branch after merging this pr.

Kudos to Jun and @susan1314 for fixing this bug!