Custom field empty entry is resulting in 'Array'

I use an Gravity Forms form to update some custom post types. In this form i have some custom fields for meta data.

• _pronamic_company_twitter
• _pronamic_company_facebook
• _pronamic_company_linkedin
• _pronamic_company_google_plus

We have checked the “Unique Custom Field? (?)” checkbox, but if we leave the fields empty it will store the string ‘Array’ in the meta data.

WordPress: 3.5.2
Gravity Forms: 1.7.6
Gravity Forms – Update Post: 0.5.3

https://www.ads-software.com/extend/plugins/gravity-forms-update-post/