reCaptcha problem in Form Creator

I am trying the reCaptcha feature for the first time.

However, when I generate my form and add my keys in the plugin, the form that is generated shows the reCaptcha widget at the bottom of the page saying:

ERROR for site owner: Invalid key type

I am using the same keys I have used on a user-signup form that I hand-coded myself and where reCaptcha works fine.

Any ideas?

Thanks.