Custom field with Checkbox list

Hello Sitefinity Team,

I wanted to create a custom field using CheckBoxes as  the Option for the Multiple choice. However, I am not able to select more than one item from the checkboxes list in the documents & files content section. 

while selecting multiple type, i am getting the below error :

"There was an error deserializing the object of type Telerik.Sitefinity.Web.Services.ContentItemContext`1[[Telerik.Sitefinity.Libraries.Model.Document, Telerik.Sitefinity.Model, Version=8.2.5900.0, Culture=neutral, PublicKeyToken=b28c218413bdf563]]. End element 'UserType' from namespace '' expected. Found element 'item' from namespace ''."

How do I create a custom field with checkboxList as an option, so that i can select multiple items at once, as per my requirement.

Please suggest.

I am using Sitefinity 8.2 version

Thanks,

Alka