Exception when running spectral 6.4.0 or later with specific custom rule using js function: endsWith is not a function

[phoenixy1]

Describe the bug
A custom rule that worked in spectral 6.3.0 and earlier is causing spectral to throw an exception in 6.4.0 and later
To Reproduce

1. Given this OpenAPI/AsyncAPI document: https://github.com/plaid/plaid-openapi/blob/master/2020-09-14.yml
2. With the following custom rule in the spectral ruleset:

  response-object-additional-properties:
    description: Every model of type object used in a response has additionalProperties=true
    message: '{{path}} {{error}}'
    severity: error
    given: '$.components.schemas.[?(@.type == ''object'')]'
    resolved: false
    then:
      function: response-object-additional-properties

and the following response-object-additional-properties.js function defined:

module.exports = (input, options, context) => {
    if (context.path == null) {
        return;
    }

    const parentPropertyName = context.path[context.path.length - 1]
    if (parentPropertyName.endsWith('Response')) {
        if (!input.additionalProperties) {
            return [{
                message: 'No additional properties field',
            }]
        }
    }
}

4. Run this CLI command: spectral lint 2020-09-14.yml
5. Error received:

Error #1: Function "responseObjectAdditionalProperties" threw an exception: parentPropertyName.endsWith is not a function
Error #2: Function "responseObjectAdditionalProperties" threw an exception: parentPropertyName.endsWith is not a function
Error #3: Function "responseObjectAdditionalProperties" threw an exception: parentPropertyName.endsWith is not a function

Expected behavior
I expected spectral to run without an error.

Additional context
This error appears in Spectral 6.4.0 and later. Spectral 6.3.0 does not exhibit this issue.