Method does not exist or incorrect signature: void getHeaderKey(String) from the type System.HttpResponse

I'm simply trying to access an Http response header key value as specified in the docs,

• public String getHeader(String key)
  • https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_restful_http_httpresponse.htm⌗apex_System_HttpResponse_getHeader

When I try to save, the compiler is complaining about the method signature. I've tried in a few different orgs to make sure somehow the HttpResponse class hadn't been overridden somehow...

Thanks for any clues/suggestions.

public class HttpTesting {
    public HttpTesting() {
        String aString = 'aHeaderKey';
        HttpRequest anHttpRequest = new HttpRequest();
		HttpResponse anHttpResponse = new Http().send(anHttpRequest);
        System.debug('---> anHttpResponse.getHeaderKeys(): ' + anHttpResponse.getHeaderKeys());
        System.debug('---> anHttpResponse.getHeaderKey(aString): ' + anHttpResponse.getHeaderKey(aString));  // <--- 'Method does not exist or incorrect signature: void getHeaderKey(String) from the type System.HttpResponse'
    }
}