Error "null" using Flow to send email with "Sender Type: OrgWideEmailAddress"

This is a weird error on sending emails using Flows, so any advice would be appreciated.

Setting up a Flow to automatically send out emails to external users from a sales person in the when Opportunities hit conditions for renewals.  I'm using the Send Email Action, using an email template, I set Sender Type to OrgWideEmailAddress and set the sender's email, when I test the flow using Debug, I get an Error that says "null".  

I already set up the sender in Organization-Wide Email Addresses and he already got the verification email and hence, the status is "Verified".  When I don't set OrgWideEmailAddress and use that person's email address as the Sender, that setting is ignored and it just sends an email with my name from the salesforce org (it makes sense, the Sender is only required when including the Sender Type and as the admin I'm the default user of the flow).    

 I also tried other things like putting the user's email or user ID in a variable and putting that variable in the Sender field, which also didn't work: "Error Occurred: Org-Wide Email provided is not valid.  Please fix and try again."  

Any ideas on why the Sender is being disregarded/read as null when the Sender Type is set to OrgWideEmailAddress?  Previous admins used a work flow and email plugin for outside emails, which I'm trying to not use since workflows aren't supported and flows are more capable.  Let me know what you think.  Thanks.

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