Cannot retrieve access_token -- no errors, just blank result

Hi,

First time trying to connect, so hopefully this isn't dumb.

1. Using V3 API
2. Have created an account
3. Have created an application
4. Have created an API Key / Client ID
5. Have created a secret
6. Have created a formatted URL, which successfully produces an authorization code after I login and allow it
7. I plug these values into the getAccessToken() function from https://v3.developer.constantcontact.com/api_guide/server_flow.html:
   • RedirectURI matches exactly what's in the application portal
   • ClientID matches exactly what's in the application portal
   • ClientSecret matches exactly what's in the application portal
   • Code matches exactly the authorization code returned in step 6 above
   • I'm doing all this within 60 seconds of generating the authorization code
8. I'm expected to be given an access_token and refresh_token
9. Result:  blank.  I echo the result and there's nothing there.

Feels like I must be doing something obviously wrong, but can't figure out what?

Thanks in advance for any guidance.

Steve