Fixes#12589.
r? @Manishearth
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#12589
- [x] There are tests for these changes OR
Source-Repo: https://github.com/servo/servo
Source-Revision: b2f69f363574d5dea3cb96d4ef00c1d4e56bdd63