Form fields not appearing + 3x errors

On 2x websites I run the Mailchimp sign up form isn't working on either. The submit button appears but no fields, and when you press submit it gives the following errors:

Notice: Undefined index: mergevars in mailchimp_signup_subscribe_form_submit() (line 362 of /home/m18m18/public_html/sites/all/modules/mailchimp/modules/mailchimp_signup/mailchimp_signup.module).
Warning: array_filter() expects parameter 1 to be array, null given in mailchimp_signup_subscribe_form_submit() (line 362 of /home/m18m18/public_html/sites/all/modules/mailchimp/modules/mailchimp_signup/mailchimp_signup.module).
Warning: Invalid argument supplied for foreach() in mailchimp_signup_subscribe_form_submit() (line 364 of /home/m18m18/public_html/sites/all/modules/mailchimp/modules/mailchimp_signup/mailchimp_signup.module).

I've logged in to the Mailchimp account and the API key, subscriber list etc are all correct. It did work until recently.

Kind regards
Liam