Receiving "Salesforce Could Not Create This Lead" Email

All of my forms on the web site are working through this app with one exception. The only difference between the form that is not working and the other forms is that this form asks for an address, including country, but none of the address info is required. I also created the field as a text field, so there is no drop-down as referenced in the email below. I appreciate any assistance to get my client’s form working properly. Thank you in advance.

Here is the email I received from SalesForce verbatim:

Salesforce could not create this lead because of the reason listed below. For more information about this error or help with Web-to-Case-Lead, please contact Customer Support.

Reason: There’s a problem with this country, even though it may appear correct. Please select a country from the list of valid countries.
Lead Capture Page: Not available.

Record Information: (I have deleted certain fields here for privacy)

city = XX
company = XX
country = USA
description = Test Number 2
email = XX
encoding = UTF-8
encoding = UTF-8
event_end = 2015-05-18
event_start = 2015-05-15
first_name = Sharon
last_name = XX
lead_source = Lead form on XX
oid = 00D1a000000KTzz
orgid = 00D1a000000KTzz
phone = XX
state = SC
street = XX
zip = 00

To incorporate this lead into salesforce.com you can key in the data above.

If you have any questions, please click on Support at the top right of any page within salesforce.com.

https://www.ads-software.com/plugins/salesforce-wordpress-to-lead/