Unicode error while creating a mock server from OpenAPI yaml

When attempting to create a mock server from an OpenAPI yaml file that has a non-ASCII-printable unicode character in it, I get the following error:

Invalid Schema supplied

Invalid format. Input must be in YAML or JSON format. Specification is not a valid YAML. YAMLException: the stream contains non-printable characters at line …, column …

AFAICT, unicode characters are perfectly valid for UTF-8 encoded OpenAPI yaml files, per spec.

Is this a bug, or am I misunderstanding? Thanks!