🐛 Guard before type assertion

[cbandy]

Without this, Go type aliases cause a panic.

Fixes #813

[k8s-ci-robot]

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[sbueringer]

/ok-to-test

[cbandy]

This is another place that type aliases break.

[sbueringer]

Thank you!

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: ffda7cdcca33d6b15ede0596b9091bce8f620bf5

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cbandy, sbueringer

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