Checkbox field empty from web to lead form

I have a custom field, type is checkbox which is being used in a WebToLeadForm. 

All input field data passes into the lead record correctly, except for the checkbox. After Submit, it is passed in the query string as

email_optin_c=checked

The HTML for the input it: 

<input id="email_opt_in_c" name="email_optin_c" type="checkbox" value="checked" />

The settings on the field type in studio are: 

Data type: Checkbox

Field Name: email_opt_in_c

Display Label: Email Opt-in

System Label: LBL_EMAIL_OPT_IN

Help Text: not set

Comment Text:not set

Default Value: not set

Calculated Value:not set

Dependent: not set
Reportable: yes
Audit:  not set
Allow Imports: Yes
Duplicate Merge: Enabled

Appreciate assistance on how to have the checkbox updated from the web to lead form.